Preheat & Prep: Preheat oven to 375°F (190°C). Grease an 8x8 inch or 9-inch round baking dish.
Make the Filling: In a large bowl, add the sliced apples, maple syrup, lemon juice, cinnamon, nutmeg, and arrowroot/cornstarch (if using). Toss gently until the apples are evenly coated. Pour the apple mixture into your prepared baking dish and spread it out evenly.
Make the Topping: In a separate medium bowl, whisk together the rolled oats, almond flour, optional nuts, coconut sugar/maple syrup, cinnamon, and salt. Pour in the melted coconut oil or ghee. Use a fork to mix until the mixture is crumbly and no dry spots remain.
Assemble: Sprinkle the crumble topping evenly over the apple filling in the baking dish.
Bake: Place the dish on a baking sheet to catch any drips. Bake for 40-45 minutes, or until the topping is deep golden brown and the fruit filling is bubbling around the edges.
Cool & Serve: Remove from the oven and let the crumble cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set. Serve warm, topped with a scoop of vanilla yogurt or ice cream if desired.