Preheat the oven to 425 °F (or 220 °C).

Prepare the vegetables: Dice onion, carrots, celery, and potatoes into bite-sized pieces.

Cook base: In a large oven-proof skillet or Dutch oven, melt 3 tablespoons of butter over medium heat. Add the onion and celery; sauté about 5 minutes until softened.

Make the roux: Stir in the flour, salt, garlic powder, and pepper. Cook 1–2 minutes, stirring constantly, to cook off the raw flour taste.

Create the sauce: Slowly whisk in the chicken broth and heavy cream. Keep stirring until the mixture thickens, which should take around 10 minutes.

Add the protein and vegetables: Stir in cooked chicken, carrots, potatoes, and peas (if using). Remove from heat.

Mix biscuit topping: In a separate bowl, combine biscuit mix, milk, and cheddar cheese. (Set aside the seasoning packet.)

Top and bake: Drop spoonfuls of the biscuit mixture over the filling in the skillet. Place in the oven and bake 14–18 minutes (or until biscuits are golden and cooked through).

Finish with butter: While the biscuits bake, melt ¼ cup butter and mix in the reserved biscuit seasoning packet. When biscuits are done, brush the seasoned butter over the tops.

Rest & serve: Let the casserole rest a few minutes so the filling sets slightly before serving.

Tip: If you don’t have an oven-proof skillet, transfer the filling to a greased 9×13-inch baking dish, then drop the biscuit topping and bake as instructed.