Preheat oven to 375°F (175°C) and set a baking dish on a sheet pan to catch any bubble-over.
In the baking dish or oven safe skillet, toss 32 ounces mixed berries (fresh or frozen) with 1-2 tablespoons cornstarch until evenly coated. Spread into an even layer.
Break 1 package ready-to-bake cookie dough over the berries in uneven chunks — or leave the pieces whole and let them bake as is. Leave small gaps so the fruit can bubble up through the top.
Bake until the edges bubble and the cookie topping turns golden, about 30-45 mins (depending on whether you are using fresh or frozen). A little extra time is fine — it just deepens the caramelization and thickens the filling. If the cookies are getting too hard on top, you can cover them the last 10 minutes or so with foil.
Let it sit 20 minutes before serving. The filling firms up from molten to spoonable as it cools — don't skip this.