Heat the oven to 375°F and coat a 9-by-13-inch baking dish with cooking spray.
Stir the peaches, frozen raspberries, sugar and cornstarch together directly in the dish and spread the fruit level.
Sprinkle the baking mix evenly over the fruit without stirring it in.
Drizzle the milk and melted butter slowly over the dry mix, covering as much of the surface as possible.
Bake uncovered for 42 to 48 minutes, until the topping is golden and thick fruit bubbles through the center, not just the edge.
Cool for 20 minutes before serving so the juices stop running.
