Heat the oven to 375°F and set an 8-inch square baking dish on a sheet pan.
Toss apples with lemon juice, 1 tablespoon maple syrup, 1 teaspoon cinnamon, cornstarch and half the salt.
Spread the apples evenly and press lightly so large gaps do not remain.
Mix oats, almond flour, walnuts, remaining cinnamon and salt. Stir in melted butter, vanilla and remaining maple until clumps form.
Scatter topping without packing it down and bake 35 to 45 minutes, until fruit bubbles at the edges and the top is browned.
Rest 15 minutes so the juices thicken before spooning into bowls.