Preheat and prep. Heat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with butter or nonstick spray.

Add the berries. Pour the frozen mixed berries into the baking dish. Spread them into an even layer. If any large strawberry pieces are whole, break them up slightly.

Sweeten the fruit. Sprinkle the sugar evenly over the berries. Use 2 tablespoons for sweeter mixes, up to 4 if your berries are very tart.

Add the cake mix. Sprinkle the dry cake mix evenly over the sugared berries. Don’t stir. Aim for full coverage so the butter has something to soak into.

Drizzle the butter. Pour the melted butter evenly over the surface. Try to moisten as much of the cake mix as possible. It’s okay if a few dry patches remain.

Bake. Place the dish on the center rack and bake for 40–50 minutes. The top should look golden and crisp, and the berry juices should be bubbling around the edges.

Rest and serve. Let the cake cool for at least 10 minutes so the juices thicken slightly. Spoon into bowls and serve warm. Optional: add a scoop of vanilla ice cream or a dollop of whipped cream.