Preheat the oven according to the cake mix package instructions.

Combine the frozen mixed berries, brown sugar, and lemon juice in a bowl. Toss gently until coated, and let sit for a few minutes.

Mix the cake mix, softened butter, milk, and granulated sugar until well combined.

Pour half of the cake batter into a greased baking dish, spreading it evenly across the bottom.

Spread the berry mixture over the batter evenly.

Top with the remaining batter, pouring it gently over the berries.

Bake according to package instructions or until a toothpick comes out clean.

Let cool for 10-15 minutes before serving.