Prep your pan and oven: Place a rack in the lower third of the oven and preheat to 400°F (200°C). Set a baking sheet on the rack to preheat as well; it will catch any drips and help crisp the bottom crust.

Fit the crust: Roll out the pie dough if needed and ease it into a 9-inch pie plate. Trim excess, leaving about a 1-inch overhang, then fold and crimp. Chill the crust in the fridge for 20 minutes while you make the filling and crumble.

Make the filling: In a large bowl, combine peaches, raspberries, sugar, cornstarch, lemon juice, vanilla, cinnamon, and salt. Toss gently so you don’t crush the berries. Let the mixture sit 5–10 minutes to start releasing juices.

Mix the crumble: In a medium bowl, stir together flour, oats, brown sugar, cinnamon, and salt. Work in the cold butter with your fingers or a pastry cutter until the mixture forms moist clumps that hold when pressed. Stir in nuts if using. Keep the crumble chilled until you’re ready to top the pie.

Fill the crust: If using, brush the crimped crust with a light egg wash. Pour the fruit mixture into the chilled crust and dot the top with the tablespoon of butter.

Add the crumble: Sprinkle the crumble evenly over the fruit, breaking up any large chunks and covering the surface to the edges. If you like extra crunch, add a pinch of coarse sugar on top.

Bake hot to set the crust: Place the pie on the preheated baking sheet. Bake at 400°F for 20 minutes to help the bottom crust begin to set.

Reduce heat and continue: Lower the oven to 350°F (175°C) and bake 30–40 minutes more, until the crumble is golden and the filling is bubbling thickly around the edges. Tent with foil if the topping browns too quickly.

Cool: Transfer the pie to a rack and cool at least 3 hours before slicing. This rest time helps the juices thicken so the slices hold together.

Serve: Enjoy at room temperature or slightly warm. Vanilla ice cream, whipped cream, or a spoonful of Greek yogurt all pair beautifully.