1. Preheat oven to 350 degrees.
2. Rinse berries and pat dry. Slice strawberries.
3. To brown the butter, add butter for the berry filling to a medium saucepan. Heat over medium heat, stirring occasionally. Once butter melts, continue stirring often as the butter browns. It will sizzle and crackle. Do not walk away from the pan at this point. Continue stirring often until the butter is golden brown and the sizzling sound stops. It will be very aromatic. Pour the brown butter into a medium bowl.
4. Add the berries, brown sugar, cornstarch, vanilla and salt. Use a large spoon to gently mix the ingredients together until the berries are coated. Do not over mix.
5. Pour the berry mixture into the cast iron skillet.

For Crumble Topping

1. In a medium bowl combine the oats, flour, brown sugar, cinnamon and salt. Stir to combine. Add the cold, cubed butter and use your hands or a pastry cutter to mix the butter into the dry ingredients. Continue working the butter in until you have pea-sized bits of crumble that clump together. Sprinkle the crumble topping evenly over the berries.
2. Place the skillet on a large baking sheet and transfer to the oven. Bake for 40 minutes or until the berry juices are bubbling at the edges and the top is golden brown. Remove from the oven and cool slightly before serving.
3. Top with vanilla ice cream if desired.