Prep the oven and dish: Preheat your oven to 350°F (175°C). Grease a 9-inch square baking dish or similar casserole with a bit of butter or oil.

Slice the fruit: Peel and thinly slice the apples and the chayote (or zucchini). Keep slices about 1/8-inch thick for even baking.

Season the filling: In a large bowl, toss the sliced fruit with lemon juice, 1/4 cup sweetener, 1 teaspoon cinnamon, nutmeg, vanilla, and a pinch of salt. If using xanthan gum or glucomannan, sprinkle it evenly and toss again to coat.

Make the crumble: In another bowl, combine almond flour, remaining 1/4 cup sweetener, remaining 1 teaspoon cinnamon, shredded coconut (if using), chopped nuts, and a pinch of salt. Add the cold butter cubes and cut them into the mixture with a pastry cutter or your fingertips until you get coarse crumbs.

Assemble: Spread the fruit mixture evenly in the baking dish. Sprinkle the crumble topping over the fruit, covering it fully but without packing it down.

Bake: Bake for 35–45 minutes, until the topping is golden and the fruit is bubbling around the edges. If the top browns too quickly, tent loosely with foil for the last 10 minutes.

Rest and serve: Let the crisp rest for 10–15 minutes so the juices thicken. Serve warm with a spoonful of whipped cream or a scoop of low-carb vanilla ice cream.