In a mixing bowl, combine crushed graham cracker crumbs, sugar, and melted butter until the mixture resembles wet sand. Line a muffin tin with cupcake liners, then evenly press about a tablespoon of the crumb mixture into the bottom of each liner. Chill in the refrigerator for about 15 minutes to allow the crusts to set firmly before filling.

In another bowl, beat softened cream cheese and sugar together with an electric mixer until smooth and creamy, about 2 minutes. Gradually add the vanilla extract, flour, and egg, mixing on low speed just until everything is combined. Be cautious not to overmix to prevent cracks.

Peel, core, and dice the Golden Delicious apples into small cubes. In a separate bowl, toss the diced apples with lemon juice, cornstarch, cinnamon, and nutmeg. Set aside for flavors to meld.

Preheat your oven to 325°F (160°C). Take the chilled crusts from the fridge and fill each liner halfway with the cheesecake filling. On top, add a spoonful of the spiced apple mixture, pressing down gently.

Prepare the streusel by mixing flour, oats, brown sugar, melted butter, and additional cinnamon until crumbly. Sprinkle it over each mini cheesecake.

Place the muffin tin in the preheated oven and bake for 28-30 minutes, or until the edges are set and the center jiggles slightly. Keep an eye on them toward the end.

Remove the muffin tin from the oven and allow the mini cheesecakes to cool in the tin for about 30 minutes. Once cooled, transfer them to the refrigerator for at least 1-2 hours before serving.