01 - Preheat oven to 375°F and 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 to prepared baking dish and spread into an even layer.
03 - In a separate bowl, whisk together flour, ¼ cup sugar, baking powder, and salt until well combined.
04 - Add cold cubed butter to the flour mixture. Using a pastry cutter or your fingers, work the butter into the flour until mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - In a small bowl, whisk together milk and egg until blended. Pour over flour mixture and stir gently with a spatula just until combined. The dough will be thick and slightly shaggy—avoid overmixing.
06 - Drop spoonfuls of biscuit topping evenly over the fruit mixture, leaving some gaps for steam to escape. Sprinkle coarse sugar over the topping if desired.
07 - Bake for 38 to 42 minutes until topping is golden brown and fruit filling is bubbling around the edges. If topping browns too quickly, loosely cover with foil for the final 10 minutes.
08 - Remove from oven and let cool for 15 to 20 minutes before serving. Serve warm, optionally topped with vanilla ice cream or whipped cream.