Preheat oven to 400 degrees.
Place all the frozen berries into a large, oven-safe skillet, about 10-to-12 inches wide. Cut any large strawberries in half to reduce cooking time. Simmer berries until liquid reduces and juices have thickened, about 20 minutes. Stir in maple syrup and corn starch and cook for an additional 2 minutes to thicken.
While berries cook, make crumble topping. Add flour, oats, cinnamon, salt, and coconut sugar to a large bowl. Stir ingredients together to combine. Stir in maple syrup then pinch the butter into the flour mixture with your finger tips. Keep pinching and squeezing until all the butter is mixed in and it looks like wet, crumbly sand.
Once berry mixture is thick and bubbly, evenly crumble the toppping over the pan. Bake until the topping is golden brown, about 10 minutes. Enjoy warm or room temperature.