Preheat oven to 325°F (160°C). Combine graham cracker crumbs, granulated sugar, and melted unsalted butter until blended. Press mixture into muffin liners. Chill for 10 minutes.

Beat softened cream cheese with granulated sugar until creamy. Add egg, vanilla extract, and flour, mixing until just combined.

Peel, core, and dice apples. Toss with lemon juice, cornstarch, cinnamon, and nutmeg in a bowl. Set aside.

Pour cheesecake filling over crusts, filling about two-thirds. Spoon apple mixture on top and gently press down.

Combine oats, flour, brown sugar, cinnamon, and melted butter for streusel. Mix until crumbly and sprinkle over cheesecakes.

Bake for 28-30 minutes until edges are set and centers jiggle slightly. Cool in the pan for 30 minutes.

Transfer to wire rack or refrigerator. Chill for several hours or overnight before serving.

Drizzle with caramel sauce before serving. Optionally top with whipped cream or serve plain.