01 - Preheat the oven to 375°F. Lightly grease a 2-quart baking dish with butter or cooking spray.
02 - In a large bowl, combine sliced peaches, raspberries, ½ cup sugar, cornstarch, lemon juice, and vanilla extract. Gently toss until fruit is evenly coated. Transfer mixture to the prepared baking dish.
03 - In a separate medium bowl, whisk together flour, ¼ cup sugar, baking powder, and salt until well blended.
04 - Add cold, cubed butter to the flour mixture. Using a pastry cutter or your fingers, work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - In a small bowl, whisk together milk and egg until combined. Pour over the flour-butter mixture and stir gently with a spatula just until the dough comes together. Be careful not to overmix.
06 - Drop spoonfuls of the biscuit topping evenly over the fruit mixture, leaving some gaps for steam to escape. Sprinkle with coarse sugar if desired.
07 - Bake for 38–42 minutes until the topping is golden brown and the fruit filling is bubbling around the edges. If the topping begins to brown too quickly, cover loosely with aluminum foil for the last 10 minutes of baking.
08 - Remove from oven and let cool for 15–20 minutes before serving. Serve warm, optionally topped with vanilla ice cream.