Preheat oven to 325°F (160°C). Line a 12-cup muffin tin with paper liners. In a medium bowl, combine graham cracker crumbs, 1/4 cup granulated sugar, and melted butter. Mix until well combined and mixture resembles wet sand. Divide the mixture evenly among the 12 muffin cups, pressing firmly into the bottom of each liner. Bake for 5 minutes. Remove from oven and set aside to cool slightly while preparing the filling.

In a small saucepan, melt 1 tbsp butter over medium heat. Add finely diced apple, 1 tbsp granulated sugar, and 1/2 tsp cinnamon. Cook, stirring occasionally, for 5-7 minutes, or until apples are slightly softened but still hold their shape. Remove from heat and set aside to cool completely.

In a large mixing bowl, using an electric mixer, beat softened cream cheese until smooth. Gradually add 1/2 cup granulated sugar and beat until well combined and creamy. Beat in the egg, vanilla extract, and sour cream (if using) until just combined. Do not overmix.

Divide the cheesecake filling evenly among the 12 muffin cups, spooning it over the baked graham cracker crusts.

Spoon about 1-2 teaspoons of the cooled spiced apple mixture onto the center of each cheesecake.

In a small bowl, combine old-fashioned rolled oats, all-purpose flour, light brown sugar, and 1/4 tsp cinnamon. Add the cold, cubed butter. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

Sprinkle the oat crisp topping evenly over the apple topping on each cheesecake.

Bake for 20-25 minutes, or until the edges are set and the centers are slightly jiggly. Turn off the oven, prop the oven door open slightly, and let the cheesecakes cool in the oven for 30 minutes. This helps prevent cracking.

Remove the muffin tin from the oven and let cool completely on a wire rack at room temperature. Once cooled, transfer to the refrigerator and chill for at least 4 hours, or preferably overnight, before serving.