Preheat oven to 350°F (175°C). Wrap the outside bottom and sides of a 9-inch springform pan tightly with heavy-duty aluminum foil to prevent water bath leakage.

In a medium bowl, combine graham cracker crumbs, ¼ cup granulated sugar, and melted butter. Mix until well combined and crumbs are moistened.

Press the mixture evenly into the bottom of the prepared springform pan.

Bake for 10-12 minutes, or until lightly golden. Remove from oven and set aside to cool slightly while you prepare the filling. Reduce oven temperature to 325°F (160°C).

In a large mixing bowl, using an electric mixer on medium speed, beat the softened cream cheese until smooth and creamy, about 2-3 minutes. Scrape down the sides of the bowl as needed.

Gradually add 1 cup granulated sugar, 2 tbsp flour, vanilla extract, ½ tsp cinnamon, and nutmeg. Beat on low speed until just combined, scraping the bowl frequently. Do not overmix.

Add eggs one at a time, beating on low speed after each addition just until incorporated. Overmixing after adding eggs can introduce too much air, leading to cracks.

Stir in the sour cream until just combined. The filling should be smooth.

Pour the cheesecake filling over the cooled crust in the springform pan.

Place the foil-wrapped springform pan into a larger roasting pan. Carefully pour hot water into the roasting pan, ensuring the water comes halfway up the sides of the springform pan. This creates a water bath (bain-marie) for even baking.

Carefully transfer the roasting pan to the preheated oven. Bake for 60-75 minutes, or until the edges are set but the center (about 2-3 inches) still jiggles slightly when gently shaken.

Turn off the oven, crack the oven door slightly, and let the cheesecake cool in the water bath for 1 hour. This slow cooling helps prevent cracks.

Carefully remove the springform pan from the water bath and remove the foil. Let cool completely on a wire rack for another 1-2 hours.

While the cheesecake is cooling, prepare the apple filling. In a medium saucepan, melt 2 tbsp butter over medium heat. Add diced apples, ¼ cup brown sugar, 1 tbsp flour, 1 tsp cinnamon, and ¼ tsp nutmeg. Cook, stirring occasionally, for 8-10 minutes, or until apples are slightly tender but still hold their shape. Remove from heat and let cool.

In a separate medium bowl, combine old-fashioned oats, ¼ cup flour, ¼ cup brown sugar, ¼ cup granulated sugar, and ½ tsp cinnamon for the crisp topping.

Add the cold, diced butter to the oat mixture. Using your fingertips, a pastry blender, or a fork, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

Once the cheesecake is completely cooled, gently spread the cooked apple mixture over the top.

Evenly sprinkle the oat crisp topping over the apples.

Refrigerate the Apple Crisp Cheesecake for at least 6 hours, or preferably overnight, to allow it to set completely.

Before serving, carefully run a thin knife around the edge of the pan before releasing the springform collar.