Preheat oven to 350 degrees F (175 degrees C). Grease an 8x8 inch baking dish.

To make the crumble topping, whisk together the oats, flour, sugars, cinnamon, and salt in a medium bowl. Stir in the melted butter and vanilla extract until the mixture is crumbly.

To make the berry filling, combine the frozen berries, oat flour, sugar, salt, lemon juice, and vanilla extract in a large bowl. Stir until well combined.

Pour the berry filling into the prepared baking dish.

Sprinkle the crumble topping over the berry filling.

Bake in the preheated oven for 40-45 minutes, or until the topping is golden brown and the filling is bubbling.

Serve warm with vanilla ice cream.