Preheat your oven to 375°F (190°C). Rookie mistake alert: forgetting this step slows everything down, so do it first.
Prep the fruit. Toss the sliced peaches and raspberries into a large bowl. Add granulated sugar, brown sugar, cornstarch, lemon juice, vanilla, and cinnamon. Gently mix until everything looks glossy and evenly coated.
Fill the crust. Pour the fruit mixture into your unbaked pie crust. Spread it out evenly so every slice gets a fair share of peaches and raspberries.
Make the crumble topping. In another bowl, combine flour, brown sugar, oats, and salt. Add the cold butter cubes and use your fingers or a pastry cutter to work it in until the mixture looks like chunky sand. You want clumps—don’t overthink it.
Top it generously. Sprinkle the crumble evenly over the fruit filling. Don’t be shy. This topping is the whole point.
Bake the pie. Place the pie on a baking sheet (trust me on this) and bake for 45–55 minutes, until the filling bubbles and the topping turns golden brown.
Cool before slicing. Let the pie cool for at least 1–2 hours. I know it’s hard, but this helps the filling set. Warm is good. Lava-hot chaos is not.