- 10 minutes
- 25 minutes
- Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil.
- Mix flour, 1/2 cup sugar, baking powder, and salt in a bowl; mash butter into the flour mixture until crumbly.
- Dissolve cornstarch in cold water in a separate large bowl. Stir blackberries, raspberries, 3/4 cup sugar, and lemon juice into the cornstarch mixture; transfer to a cast-iron skillet and bring to a boil, stirring frequently. Drop flour mixture into the skillet by the spoonful. Put skillet onto the prepared baking sheet.
- Bake in preheated oven until the topping is golden brown, about 25 minutes.