Heat the oven: Preheat to 350°F (175°C). Lightly butter a 9x13-inch baking dish or a similar 3-quart dish.

Prep the peaches: Peel if you like (optional). Slice into 1/2-inch wedges. You’ll want about 6 cups of sliced peaches.

Mix the fruit filling: In a large bowl, combine peaches and berries with 1/3–1/2 cup granulated sugar (adjust to fruit sweetness), 2 tablespoons cornstarch, 1 teaspoon lemon zest, 1 tablespoon lemon juice, 1 teaspoon vanilla, and a pinch of salt. Gently toss until coated. Spread evenly in the baking dish.

Make the crumble topping: In another bowl, stir together 1 cup all-purpose flour, 1 cup old-fashioned oats, 1/2 cup brown sugar, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt. Cut in 1/2 cup (1 stick) cold unsalted butter, cubed. Use a pastry cutter or your fingers to rub the butter into the dry mix until it forms moist clumps with some sandy bits. For extra crunch, mix in 1/2 cup chopped nuts.

Assemble: Sprinkle the crumble evenly over the fruit. Don’t pack it down—clumps are good.

Bake: Set the dish on a lined baking sheet to catch drips. Bake for 40–50 minutes, until the topping is golden and the fruit is bubbling thickly around the edges.

Rest: Let the crumble cool for at least 15–20 minutes. This helps the juices set so each scoop holds together.

Serve: Enjoy warm with vanilla ice cream, whipped cream, or a spoonful of Greek yogurt.