Preheat the oven to 350°F
Lightly grease a glass 8x8 pan. Tilt against a wall or other stationary object, so dish is on it's side.
Wash and core the apples, then using a mandoline slicer, slice them thinly. Cut each of the slices in half, and place in the sideways glass dish so that if you were to place the dish flat on the counter, the skin side (rounded side) would face straight up.
Create roughly three piles, alternating which pile you place each slice into so that the slices slightly overlap. Repeat until all four apples are in the dish, creating three interlocking rows. Lay the dish flat on the counter, and set aside.
In a medium bowl, combine the melted butter, Truvia, and almond milk. Beat in the eggs.
Add the vanilla extract and salt, and whisk until combined.
Whisk in the almond and coconut flours. Add the baking powder, and whisk again.
Pour batter over the apples in the glass 8x8 dish, and sprinkle a light dusting of cinnamon on top, if using.
Bake in preheated oven for 30-45 minutes or until edges start to brown.
Once baked, cool almost entirely before proceeding with caramel sauce (a quick and easy but definitely worthy addition).