Preheat your oven to 350°F (175°C) and lightly grease or line your baking dish.
Make the apple filling: In a bowl, toss sliced apples with lemon juice, cinnamon, nutmeg, sweetener, and gluten-free flour or starch if using. Pour into baking dish.
Make the crumble: In another bowl, mix oats, flour, nuts, sugar, cinnamon, and salt. Stir in melted coconut oil and vanilla until crumbly.
Assemble: Sprinkle crumble topping evenly over the apple mixture.
Bake for 30–35 minutes, or until the topping is golden and apples are bubbling.
Cool for 10 minutes before serving. Serve warm with a scoop of vanilla ice cream, yogurt, or just a big spoon!