Prepare the crust: Mix graham cracker crumbs, brown sugar, and melted butter. Press into the base of each muffin tin.
Make the cheesecake filling: Beat softened cream cheese with sugar until smooth. Add eggs, vanilla, and lemon juice. Mix well.
Prepare the apple topping: Toss diced apples with sugar, cinnamon, nutmeg, and lemon juice. Set aside.
Make the crumble: Combine oats, flour, and brown sugar. Cut in cold butter until crumbly.
Assemble the cheesecakes: Layer cheesecake filling over the crust, top with apples, and sprinkle crumble on top.
Bake: Preheat oven to 325°F (163°C). Bake for 25-30 minutes, or until set.
Cool and serve: Let cool in the tin, refrigerate for at least 1 hour before serving.