Easy Raspberry Rhubarb Slab Pie with Lattice Crust

There’s a special kind of magic that happens when the first stalks of rhubarb appear at the market. It’s a sure sign that spring is here and summer is just around the corner. For me, that means it’s time to bake a pie. But not just any pie! When I need to feed a crowd for a picnic, potluck, or family gathering, I turn to this incredible Raspberry Rhubarb Slab Pie. It has all the buttery, flaky crust and tart-sweet fruit filling of a classic pie, but it’s baked in a jelly roll pan, making it easy to slice, serve, and share with everyone you love.

The combination of tart rhubarb and sweet, juicy raspberries is a match made in heaven. Brightened with a hint of orange zest and fragrant vanilla bean, the filling is vibrant and utterly delicious. This isn’t just a dessert; it’s a celebration of the season, perfect for making memories.

Table of Contents

Contents List

Why make this Raspberry Rhubarb Slab Pie

This Raspberry Rhubarb Slab Pie is a showstopper that’s surprisingly straightforward to create. Here’s why you’ll fall in love with it:

Feeds a Crowd: Baked in a large jelly roll pan, this recipe yields 12-15 generous servings, making it the perfect dessert for parties, holidays, and summer cookouts. No more fighting over the last slice!

Perfect Flavor Pairing: The tartness of the rhubarb is perfectly balanced by the sweetness of fresh raspberries. The addition of orange zest and real vanilla bean seeds elevates the filling from simple to sublime.

Flaky, Buttery Crust: This recipe uses a combination of all-purpose and whole wheat pastry flour for a crust that is both sturdy and tender, with a lovely nutty flavor. It’s the ideal vessel for the juicy fruit filling.

Stunning Presentation: The beautiful woven lattice top isn’t just for looks; it allows steam to escape while the pie bakes, preventing a soggy bottom. A sprinkle of demerara sugar adds a delightful crunch and sparkle. If you love the rhubarb and berry combination, you should also try these Strawberry Rhubarb Cheesecake Bars for another crowd-pleasing treat.

Ingredients

• For the crust
• 48 tbsp (24 oz) unsalted butter, cold, divided
• 5 cups all purpose flour, divided
• 1¼ cups whole wheat pastry flour, divided
• 6 tbsp sugar, divided
• 3 tsp fine sea salt, divided
• 1 cup water, cold, divided
• For the filling
• 1½ lb rhubarb
• 1 lb raspberries
• 2 cups sugar
• zest of 1 orange
• 2 vanilla beans
• ¼ cup + 2 tbsp arrowroot powder
• 1 tsp fine sea salt
• For baking the pie
• 1 large egg
• pinch of fine sea salt
• 2 tbsp demerara sugar

How to make Raspberry Rhubarb Slab Pie

Step 1: To start the crust, you’ll work in two batches for easier handling. For the first batch, cut 24 tbsp of cold butter into ½-inch pieces and place them in the freezer for 10 minutes. In a stand mixer with the paddle attachment, combine half of the flours, sugar, and salt. Mix on low to combine.

Step 2: Add the frozen butter pieces to the mixer and mix on low for 1-2 minutes until you have coarse crumbs with some larger butter pieces remaining. Drizzle in half of the cold water and mix just until the dough starts to come together.

Step 3: Turn the dough out onto a clean surface and pat it into a rectangle. Wrap it tightly in plastic wrap and use a rolling pin to flatten it to a ½-inch thickness. Chill this first block of dough in the refrigerator for at least 2 hours, or overnight.

Step 4: Repeat steps 1-3 with the remaining half of the crust ingredients to create a second block of dough. Chill this one as well.

Step 5: When you’re ready to assemble the pie, let both dough blocks rest at room temperature for 10-15 minutes. Grease a jelly roll pan with butter. On a floured surface, roll one block of dough into a 15×20-inch rectangle. Carefully transfer it to the prepared pan, pressing it into the corners and sides. Trim the dough, leaving a 1-inch overhang. Form the trimmings into a small block. Place the crust-lined pan and the trimmings block in the fridge.

Step 6: For the lattice top, roll the second block of dough into another 15×20-inch rectangle. Place it on a parchment-lined baking sheet and chill for 30 minutes. Meanwhile, roll the block of trimmings into a 12-inch square, place it on parchment, and chill for 30 minutes. Cut both the main lattice dough and the trimmings dough into 1-inch wide strips. Return all strips to the fridge while you make the filling.

Step 7: Prepare the filling by trimming and cutting the rhubarb into ½-inch pieces. Place it in a large bowl with the raspberries. In a separate bowl, use your fingers to rub the orange zest and vanilla bean seeds into the sugar. Whisk in the arrowroot powder and salt. Add this sugar mixture to the fruit and gently toss to combine.

Step 8: Remove the pie shell from the fridge and scoop the filling inside, spreading it evenly. Arrange 7 of the long lattice strips evenly across the pie. Weave the shorter strips through them to create a woven lattice pattern. Trim the lattice edges flush with the inside rim of the pan.

Step 9: Roll the 1-inch overhang of the bottom crust up and over the lattice edges, crimping to seal. Chill the entire pie in the refrigerator for 30 minutes. While it chills, preheat your oven to 375°F.

Step 10: Whisk the egg with a pinch of salt to create an egg wash. Brush it over the chilled pie crust and sprinkle generously with demerara sugar. Place the pie on a parchment-lined baking sheet (to catch any drips) and bake for 50-60 minutes. The crust should be golden brown and the filling bubbly. Rotate the pan halfway through baking for even color. Let the pie cool on a wire rack for at least 1 hour before slicing and serving.

How to serve Raspberry Rhubarb Slab Pie

This slab pie is absolutely divine served warm from the oven. The flaky, buttery crust and juicy, tart-sweet filling are a perfect match for a generous scoop of vanilla bean ice cream or a dollop of freshly whipped cream. For a simpler touch, a light dusting of powdered sugar just before serving also looks beautiful. Because it’s so easy to slice into squares, it’s ideal for serving a crowd without the fuss of a traditional round pie.

How to store Raspberry Rhubarb Slab Pie

Storing this pie is simple. Once it has cooled completely, you can cover the pan tightly with plastic wrap or foil. It will keep at room temperature for up to 2 days. If you prefer to store it in the refrigerator, it will last for up to 4-5 days. The crust may soften slightly in the fridge, but a quick trip to a warm oven can help re-crisp it. You can also freeze individual slices. Wrap them well in plastic wrap and then foil, and they will keep in the freezer for up to 3 months. Thaw at room temperature before enjoying.

Tips to make Raspberry Rhubarb Slab Pie

Keep it Cold: The key to a flaky pie crust is cold butter. Make sure your butter, and even your water, are very cold. Chilling the dough at every stage is crucial and prevents the butter from melting, which creates those delicious flaky layers.

Don’t Overwork the Dough: When mixing the dough, stop as soon as it comes together. Over-mixing develops the gluten in the flour, which can lead to a tough, rather than tender, crust.

Use Arrowroot Powder: Arrowroot is a fantastic thickener for fruit pies. It creates a clear, glossy filling and has a neutral flavor, unlike cornstarch which can sometimes taste starchy or appear cloudy.

Embrace the Fruit: While raspberry and rhubarb are a classic pairing, feel free to experiment! If you’re a fan of rhubarb pies, this Blueberry Rhubarb Pie – A Sweet and Tangy Dessert is another fantastic option to try.

FAQs About Raspberry Rhubarb Slab Pie

Can I use frozen rhubarb or raspberries?

Yes, you can absolutely use frozen fruit. Do not thaw the fruit before using it; simply add it to the bowl frozen and toss it with the sugar mixture. You may need to add a few extra minutes to the baking time to ensure the filling is hot and bubbly.

What is a slab pie?

A slab pie is simply a pie baked in a large, rectangular pan, typically a jelly roll or sheet pan. It has a higher crust-to-filling ratio than a deep-dish pie and is perfect for serving a large group because it’s easy to slice into many portions.

Why do you use whole wheat pastry flour in the crust?

Whole wheat pastry flour adds a wonderful, subtle nutty flavor and a more tender texture to the crust compared to regular whole wheat flour. It complements the fruity filling beautifully. If you don’t have it, you can substitute it with more all-purpose flour, but it’s worth seeking out for its unique contribution.

Final Thoughts

Baking this Raspberry Rhubarb Slab Pie is a rewarding experience from start to finish. The process of making the dough, weaving the lattice, and watching it bake to golden perfection is almost as satisfying as the first bite. It’s a dessert that brings people together, fills your home with the most wonderful aroma, and perfectly captures the bright, hopeful flavors of the season. I hope you and your loved ones enjoy every last crumb!

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