Preheat oven to 350° F and have an 8x8 or 9x9-inch pan ready. In a large bowl, mix together the 2-3 apples, 1-2 cups blackberries, ½ cup granulated sugar, 1 tablespoon arrowroot powder, and 1 teaspoon lemon juice if using. Pour into the baking pan.
In a medium bowl, mix together 1 cup oat flour, 2 cups old-fashioned rolled oats, ½ cup light or dark brown sugar, 1 teaspoon ground cinnamon, and ¼ teaspoon fine sea salt. Stir in the ½ cup unsalted butter. Spread the topping evenly over the filling mixture.
Place the crisp in the oven and bake for 45-50 minutes, checking then to see if the fruit is bubbling and the topping looks crisp. If not, check every five minutes thereafter until done. Remove crisp from oven and let cool on a rack for at least 30 minutes. Serve warm or at room temperature.
Notes
CN's Tips
Use all-purpose or gluten-free flour: If you wish to use all-purpose flour or a 1:1 gluten-free flour substitute, like my homemade gluten free flour blend, use 1 cup instead of the oat flour.
Make it refined sugar-free - Substitute coconut sugar or maple syrup for the brown sugar.
Dairy-free - Use melted non-dairy butter or melted coconut oil.
Adjust the amount of sweetener - You can reduce the amount of sugar in the filling to zero if you are using sweet apples and berries and want to go low sugar. Or, simply reduce the amount called for by ¼ cup.
Honey - Instead of granulated sugar, sweeten the filling with ½ cup of honey.
Other berries - Blueberries and strawberries also pair wonderfully with apples! You can try them in place of the blackberries if you wish.