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.
Notes
How to lighten it up:
Less cookie, more berries. Cut the cookie-dough topping and pile on extra fruit. More volume, more fiber, less sugar per bite.
Use low-sugar cookies. Swap in a low-sugar or sugar-free cookie for the topping and you slash the added sugar without touching the texture.
Dial back added sugar. Ripe berries are sweet on their own — you can usually cut the sugar by a third and never miss it.
Serve with Greek yogurt instead of ice cream for a protein bump.