Preheat oven to 375°F.
Mix the berries, flour, and sugar together in a bowl and transfer them to an 8″ or 9″ cast iron skillet (or 8” square baking dish or 9″ pie pan) and prepare your toppings.
Mix together the dry ingredients.
Add the butter, working it into the mixture until incorporated.
Sprinkle the dry ingredients over the berries.
Bake in at 375°F for 25 to 45 minutes, until the berries are bubbling and the topping has started to brown (bake the shorter amount of time for fresh berries, longer for frozen).