01 - Preheat oven to 375°F. Lightly grease a 2-quart baking dish with butter or cooking spray.
02 - Combine sliced peaches, raspberries, ½ cup sugar, cornstarch, lemon juice, and vanilla extract in a large bowl. Gently toss until fruit is evenly coated. Transfer mixture to the prepared baking dish, spreading evenly.
03 - Whisk together flour, ¼ cup sugar, baking powder, and salt in a medium bowl until well combined.
04 - Add cold, cubed butter to the flour mixture. Use a pastry cutter or work butter in with your fingers until the mixture resembles coarse crumbs with pea-sized butter pieces remaining.
05 - Whisk milk and egg together in a small bowl until blended. Pour over the flour-butter mixture and stir with a spatula just until combined. Do not overmix—small lumps are fine.
06 - Drop spoonfuls of the biscuit topping evenly over the fruit mixture, leaving some spaces for steam to escape. Sprinkle coarse sugar over the topping if desired for extra crunch.
07 - Bake for 38 to 42 minutes until the topping is deep golden brown and the fruit filling is bubbling around the edges. If the topping begins to brown too quickly, loosely cover with aluminum 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.