Preheat the oven to 350°F. Coat a medium baking dish or casserole dish with cooking spray. Set aside.
In a large bowl, toss the apple slices with lemon juice, sugar and apple pie spice.
Transfer to the baking dish and set aside.
In the same mixing bowl (no need to make another one dirty), stir together the almond flour with brown sugar, salt, apple pie spice, baking powder and oats.
Add the eggs and vanilla extract, mix until uniformly wet.
Using a spoon, dollop on top of the apple mixture until fully covered.
Pour the melted butter over the oat topping.
Cook, uncovered, for 30 minutes or until the top is lightly browned and puffy.
Remove and allow to cool for 20 minutes before serving.
Serve with vanilla ice cream, whipped cream or caramel sauce.
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Notes
*Almond flour can be substituted for gluten free all-purpose flour, regular all-purpose flour or coconut flour.