Line a (8 by 8 inch or similar) brownie pan with parchment. Preheat the oven to 350 F (180c).
Warm the nut butter and maple syrup in a pan or microwave until warmed. Mix well to combine. Add the rest of the ingredients and mix. You will get crumbly mixture. Sprinkle some water in if needed to make it a bit more clumpy (depends on the nut butter moisture content). Transfer to the pan and press evenly to make a crust.
Toss the apple slices in flour and arrange on the crust. (You can put the apple on top of the custard as well, or have 2 layers, one above the crust and one above the custard.)
Blend all the custard ingredients until well combined. Taste and adjust sweetness and flavor. Add some lemon zest for a lemony custard. Pour over the apples.
Mix the cinnamon and sugar and sprinkle on top. Bake for 45 to 55 mins. Let cool for 15 mins, then remove from pan. Chill for an hour before slicing.
Store: Refrigerate for upto 5 days. Freeze for upto a month.
