Preheat oven to 375°F. Grease an 8-inch square baking dish or 7-inch x 11-inch baking dish.
Place the mixed berries into the greased baking dish; sprinkle sugar and cinnamon on top and set aside.
In a small bowl, combine the oats, flour, brown sugar and salt. Cut in butter using a fork until mixture resembles coarse crumbs. Sprinkle the streusel mixture over berry mixture.
Bake until the crisp topping is lightly brown, and filling is bubbly, about 35-40 minutes. Serve warm with a scoop of ice cream, if desired.
Storage: Store the cooled mixed berry crisp leftovers in an airtight container. They will last about 7 days in the fridge. Serve cold, at room temperature, or re-warmed in the microwave or oven.Make ahead: You can make the crisp topping in advance. Store in the fridge for up to 3 days or freeze for up to a month.Substitutes: For best results, follow the recipe as is. However here are some common substitutes that would work well in this recipe.
Instead of granulated sugar, you can use coconut sugar, honey or maple syrup
To make it vegan, use vegan butter or coconut oil
To make this gluten-free, use an all-purpose gluten-free flour mix and gluten free rolled oats. Do not substitute only almond flour or only coconut flour.
If you don't have cornstarch for the filling, substitute it with all purpose flour.
Nutrition: Please note that the nutrition label provided is an estimate based on an online nutrition calculator. It will vary based on the specific ingredients you use. This information should not be considered a substitute for a professional nutritionist’s advice. The nutrition value is for one serving without the ice cream.