Preheat the oven to 325°F (165°C). Line approximately 20 muffin cups with paper liners.

In a medium bowl, combine the graham cracker crumbs and ¼ cup melted butter. Stir until the crumbs are evenly moistened.

Divide the crust mixture evenly among the muffin liners. Press it firmly into the bottom of each cup using the back of a spoon or the bottom of a small glass.

Bake the crusts for approximately 8 minutes, until they appear set and dry in the center.

Remove from the oven and reduce the temperature to 300°F (150°C).

Add the softened cream cheese to a large mixing bowl. Beat with an electric mixer until smooth and creamy.

Add the granulated sugar, sour cream or Greek yogurt, eggs, lemon juice, and vanilla.

Beat on medium speed just until smooth and evenly incorporated, about 1–2 minutes. Avoid excessive mixing once the eggs have been added.

Divide the cheesecake mixture evenly among the prepared muffin cups, filling them almost to the top.

Sprinkle the finely chopped Granny Smith apple evenly over each cheesecake.

To prepare the streusel, combine the flour, oats, cinnamon, and brown sugar in a medium bowl.

Add the melted salted butter and stir with a fork until coarse, buttery crumbs form.

Sprinkle the streusel generously over the chopped apples.

Bake the Apple Crisp Cheesecakes for approximately 20–22 minutes, or until the edges appear set while the centers still have a slight jiggle.

Remove the muffin pans from the oven and allow the cheesecakes to cool in the pans for 15–20 minutes.

Carefully transfer them to a wire rack and allow them to cool completely.

Refrigerate for at least 3–4 hours, preferably overnight, before serving. This allows the cheesecake filling to fully set and develop its creamy texture.