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 and spread into an even layer.
03 - In a medium 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 dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - In a small bowl, whisk together milk and egg until blended. Pour over the flour mixture and stir with a spatula just until combined. Do not overmix; a few small lumps are acceptable.
06 - Drop spoonfuls of the biscuit topping evenly over the fruit mixture, leaving small 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 10–15 minutes before serving. Serve warm, optionally topped with vanilla ice cream.