Preheat your oven to 350°F (175°C). Lightly coat an 8×8-inch baking dish with cooking spray.
Place your sliced apples in a large mixing bowl and toss with lemon juice to prevent browning.
In a small bowl, combine the sugar substitute, cinnamon, nutmeg, and cornstarch, mixing well to eliminate any lumps.
Sprinkle this mixture over the apples and add vanilla extract, then toss until all slices are evenly coated.
In a medium bowl, combine the rolled oats, whole wheat flour, almond flour, brown sugar substitute, cinnamon, and salt.
Add the cold diced butter and use a pastry cutter or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Stir in the applesauce and fold in nuts if using.
Transfer the apple mixture to your prepared baking dish, arranging in an even layer.
Sprinkle the crisp topping evenly over the apples, making sure to cover the entire surface. Avoid packing down the topping.
Bake in the preheated oven for 40-45 minutes, or until the filling is bubbling around the edges and the topping is golden brown.
If the topping browns too quickly, loosely cover with aluminum foil for the remaining baking time.
Allow the crisp to cool for at least 15 minutes before serving.