Preheat oven to 350 F.
Add frozen berries, lemon zest, and lemon juice to a saucepan. Heat over medium heat until berries are defrosted.
Add in sugar and arrowroot starch to berries and cook until thickened, about five minutes, stirring regularly.
While berries cook down, add oats, almond flour, arrowroot starch, coconut sugar, baking powder, and salt to a bowl.
Melt butter in the microwave and stir in vanilla extract. Pour into dry ingredients for crumble. Mix until combined, knowing that it will be crumbly.
Add berries to an 8x8” baking dish. Coat with crumble topping and bake for 30 minutes or until topping is golden.