Place apples in cake pan in concentric circles.
Pour brown sugar butter mixture over apples.
Add cake batter to cake pan. Smooth out top with offset spatula.
Bake in lower rack of the oven for 50 minutes to 60 minutes. Put tin foil over cake at 30 minutes, as almond flour burns easily. Check at at 40 minutes, and again at 50 minutes to check for doneness. Cake will be able to bounce when touched. Inside middle should be at 210 degrees F.
Allow to cool completely before removing cake. Use a knife to go around the sides.
Using a flat plate, turn over so that apples are the top of the cake. If using a removable bottom pan, release that first, before turning over.
If the top of cake isn't caramelized enough once you turn the cake over, place in broiler for five minutes. Watch carefully.
Cool completely before serving.
Cake can be kept in refrigerator up to one week, can be frozen.