Preheat the oven to 375 degrees.
Make the filling: In a large bowl, mix together the apple chunks, maple syrup, cinnamon, cornstarch, and lemon juice until the apple pieces are all coated.
Make the topping: In a medium bowl, mix together the oats, almond flour, cinnamon, and salt. Make sure there are no large clumps of sugar or almond flour.
Add the melted butter to the dry topping ingredients and stir to combine.
Spray an 8x8 inch baking dish with cooking spray and pour the filling into the baking dish,
Evenly sprinkle the topping over the filling.
Bake for 35 minutes, until the top is golden brown.