To make the filling, add blueberries, blackberries, cassava or tapioca flour, sweetener, water and lemon juice together in a small pot over medium heat. Turn to medium heat, and allow to simmer until it thickens to a pie filling consistency, which will take approximately 6-8 minutes. Be sure to stir frequently so it does not burn.
Spoon filling into the pre-baked pie crusts. Return to the oven for another 15-20 minutes, or until the edges of the crust begin to brown.
Remove from oven and allow to cool fully before serving or moving to an air-tight container for storage up to 5 days.
Nutrition Facts (per pie, recipe makes 8 mini pies): 74 calories, 4 g fat, 13 g total carbs, 3 g sugar alcohols, 2 g fiber (8 g net carbs), 2 g protein