1-First Step: In a large bowl, combine the 4 cups frozen mixed berries, 3 tablespoons granulated sugar, 3 tablespoons all-purpose flour, lemon zest, and a pinch of salt. Stir gently until the berries look evenly coated. Because the berries are frozen, you do not need to thaw them.

2-Second Step: Pour the berry mixture into your prepared baking dish and spread it into an even layer. If you are using one berry type, it still works, but mixing berries gives a more complex flavor. For the best results with frozen berries, keep the filling layer thick and even so it bakes uniformly.

3-Third Step: Make the oat crumble topping. In another bowl, mix 1 cup old fashioned rolled oats, 1/2 cup brown sugar, and 6 tablespoons all-purpose flour. Add the 1/2 cup butter cut into small cubes. Use your fingertips or a pastry cutter to rub the butter into the dry ingredients until you get a crumbly texture with some small clumps.

4-Fourth Step: Sprinkle the crumble topping evenly over the berry filling. Press lightly only if you see loose patches. The goal is to let the topping bake with airflow so it turns crunchy on top.

5-Final Step: Bake for 40 to 45 minutes, until the top is golden and the filling is bubbling around the edges. Let it cool for about 10 to 15 minutes before serving so the filling thickens slightly. Serve warm, and if you want extra comfort, pair it with vanilla ice cream or yogurt.