Preheat oven to 350°F (175°C).
Mix almond flour, melted butter, and egg into a dough. Press into a 9-inch tart pan, going up the sides.
Bake the crust 10 minutes, then set aside.
Melt butter in a skillet over medium heat. Add sliced chayote, sweetener, cinnamon, and lemon juice.
Cook 15–20 minutes, stirring occasionally, until the chayote is tender and most of the liquid has evaporated.
Spread the filling into the pre-baked crust.
Bake 20–25 minutes until the crust is golden.
Cool completely before slicing.