Grind walnuts, oats and flax seeds into a crumbly flour in a food processor.
Add the rest of the crust ingredients and process until it lumps together in a sticky mass; it should be able to hold its shape when pressed between two fingers.
Press into the bottom and sides of a lined tart tin or 9" springform pan and put in the fridge.
Caramel:
Blend all the ingredients together until smooth; it should be thick like yogurt.
Cut the apples into halves and core them, then slice thinly on mandoline slicer. The slices should be thin enough to bend without breaking.
Add caramel to apple slices in a big bowl and rub the caramel into them until they are all evenly coated. You may have leftover caramel.
Let them sit for 30 minutes to soften.
Fill your crust with the apples and let sit in the fridge overnight.
