Preheat the oven to 350°F (175°C) and grease 2 one-cup ramekins.
In a small mixing bowl, stir together the flour, rolled oats, cinnamon, brown sugar and salt. Add the melted butter or coconut oil and mix until combined. Set aside.
2 tablespoons (15 grams) all-purpose flour, 3 tablespoons (16 grams) rolled oats, 1/2 teaspoon cinnamon, 17 grams brown sugar, pinch salt, 1 1/2 tablespoons (21 grams) melted unsalted butter
In a medium mixing bowl, mix together the maple syrup, vanilla extract, cornstarch, cinnamon, nutmeg and salt until well combined.
2 tablespoons (40 grams) maple syrup, 1 teaspoon vanilla extract, 1 teaspoon cornstarch, 1/4 teaspoon cinnamon, 1/8 teaspoon nutmeg, pinch salt
Add the apple chunks and stir until evenly coated.
2 cups (220 grams) apple chunks
Divide the apple filling evenly between the prepared ramekins. Sprinkle half of the crumble topping over each one.
Place the ramekins on a baking sheet to catch any potential spills.
Bake for 22-26 minutes, until the topping is golden brown and the apple filling is bubbling.
Let the crumbles cool for a few minutes before serving warm with vanilla ice cream and caramel sauce, if desired. They can be made up to 3 days ahead of time. Let them cool completely, then cover and refrigerate them in their ramekins. When you're ready to serve, reheat them in a preheated 350 °F (175 °C) oven until warmed through. The topping will soften somewhat during storage, so they're best enjoyed on the day they're made. Add the ice cream and caramel sauce just before serving.