Preheat the oven to 350°F and have a 8"x8" pan ready.
Mix together the 2 cups cranberries, 6 cups sliced apples, ¾ cup brown sugar, 3 tablespoons arrowroot, 1 teaspoon ground cinnamon, and ¼ teaspoon ground ginger until well combined in the baking dish. Spread out evenly.
To make the topping, stir together the 1 cup oat flour, ¾ cup rolled oats, ¾ cup brown sugar, ½ teaspoon salt, and 1 teaspoon baking powder until combined. Cut in the ½ cup unsalted butter with a fork or your hands until the topping is crumbly.
Sprinkle the topping evenly over the filling. Place the crisp in the oven and bake for 40-45 minutes, or until the topping is browned and the filling is bubbly.
Notes
Oat flour substitute - Use sorghum or teff flour, or try a gluten-free all-purpose flour blend.
Arrowroot substitute - Swap in cornstarch equally for the arrowroot.
Make it dairy-free - Use cold coconut oil in place of the butter if you wish to make this crisp without dairy.
Make it refined sugar-free - Substitute coconut sugar for the brown sugar. Sometimes I'll do a mix of maple syrup and brown sugar in the filling for a bit of extra flavor.
Add nuts - Sliced almonds added to the topping are lovely and add a little crunch.
Add coconut - Stir in 1 cup of unsweetened, shredded coconut to the filling for a different flavor note.