Step 1: To make the pastry, add the flour, salt, and cold butter to a food processor and pulse until the mixture resembles fine breadcrumbs. Alternatively, rub the mixture together with your fingertips.
Step 2: Add the whisked egg and mix on low until the dough comes together. If it seems too dry, add cold water 1 teaspoon at a time.
Step 3: Form the dough into a ball, cover it, and chill in the fridge for 15-30 minutes.
Step 4: Preheat your oven to 170°C Fan or 190°C. Place the sliced rhubarb in a roasting tray and bake for 5 minutes to soften. Remove from the oven and drain any juices.
Step 5: While the rhubarb bakes, roll out the chilled pastry on a lightly floured surface. Gently place the pastry into your tart case, prick the base with a fork, and return it to the fridge.
Step 6: To make the filling, beat the softened butter and sugar in a bowl until light and fluffy.
Step 7: Add a spoonful of ground almonds and the eggs, and beat together. Gently mix in the remaining ground almonds and the almond extract until fully incorporated.
Step 8: Remove the pastry case from the fridge and spread the jam evenly over the base.
Step 9: Gently spoon the frangipane mixture over the jam and level the top. Trim the excess pastry from the edges of the tart case.
Step 10: Arrange the softened rhubarb in a pattern over the frangipane and scatter the sliced almonds on top.
Step 11: Bake for 35-45 minutes or until the filling is golden and set. The filling will rise and then settle as it cools.
Step 12: Allow the tart to cool completely on a wire rack before removing from the tin, slicing, and serving.