Prep the berries: In a bowl, combine mixed berries, sugar, lemon juice, and cornstarch. Toss gently until everything is evenly coated.
Add to skillet: Pour the berry mixture into a 10–12 inch cast iron skillet and spread evenly.
Make the topping: In another bowl, whisk together flour, sugar, baking powder, and salt. Cut in the cold butter using your fingers or a pastry cutter until the mixture looks like coarse crumbs.
Top the berries: Sprinkle the topping over the berry mixture. Don’t press it down keeping it loose gives that perfect texture.
Bake: Slide the skillet into a pizza oven at around 375–400°F (190–205°C). Bake for 25–30 minutes, rotating if needed, until the berries are bubbling and the top is golden brown.
Serve: Let cool 5–10 minutes. Serve warm with vanilla ice cream or whipped cream.