Preheat your oven to 375°F. Layer the apple slices in a square 9-inch baking dish, in one or two layers. You can also use four individual baking dishes.
In a medium bowl, using a fork, mix together the chopped walnuts, melted butter, cinnamon, vanilla, and (if using) sweetener.
Using a rubber spatula, pour the mixture over the apples, spreading it evenly.
Bake the coated apples until the walnut topping is golden and the apples are tender, 25-30 minutes.
Divide the apple crisp among four plates. Drizzle with the pan juices and serve.