Preheat oven to 350°F/180°C. Lightly toast almond flour in a skillet or pan over medium heat, until fully golden and fragrant (2-4 minutes). This is very important taste-wise, so don't skip!
Whisk thoroughly together in a small bowl the toasted almond flour, coconut flour, sweetener, cinnamon and salt. Add in butter and rub the mixture with your fingertips until it resembles coarse bread crumbs. Incorporate pecans and set aside.
Mix together, in a rimmed baking tray or dish, chayote, sweetener, cinnamon, nutmeg, allspice and lemon juice (starting with 1 tablespoon, tasting and adding more as needed). Add in apple extract, starting with just 1/2 teaspoon, adding more to taste. You can alternatively cook the filling a bit on the stove-top to nail the flavors (my suggestion if doing a mock apple pie!).
Top with crumble mixture and bake for 25 to 27 minutes, covering the crumble with foil after 10 minutes to avoid excessive browning.
Allow to cool for at least 15 minutes to allow the flavors to mingle, or store refrigerated for up to 3 days.
Please see post for deets on sweeteners and possible subs!