Preheat your oven to 350°F (175°C).
In a large bowl, toss the frozen berries with granulated sugar, cornstarch, and lemon juice. Spread the mixture evenly in a baking dish.
In another bowl, stir together oats, flour, brown sugar, cinnamon, and salt. Pour in the melted butter and mix until crumbly.
Sprinkle the oat mixture evenly over the berries.
Bake for 40–45 minutes, until the topping is golden brown and the berry filling is bubbling at the edges.
Let cool slightly before serving. Enjoy with a scoop of vanilla ice cream if you like.