Place a 10-inch cast iron skillet in the oven and preheat oven to 350°.
Stir together the raspberries, 1/2 cup granulated sugar, the brown sugar, 2 tablespoons flour, the lemon juice, and nutmeg in a medium bowl.
Melt the butter in the skillet in the oven until it starts to brown around the edge and foam. It’s important to have the butter sizzling hot!
While the butter is melting, stir together the remaining 1/2 cup flour and granulated sugar, the baking powder, salt, and milk in a large bowl, just until combined. The batter may still have a few lumps, but it should be about the consistency of pancake batter.
Pour the batter over the butter in the hot skillet. Do not stir. Immediately spoon the berry mixture on top of the batter. Bake until the crust looks golden and crisp, 30 to 35 minutes.