In a mixing bowl, combine graham cracker crumbs, melted unsalted butter, and granulated sugar until the mixture resembles wet sand. Press this mixture into the bottom of a 9-inch springform pan. Bake at 325°F (160°C) for 10 minutes, then let it cool completely.

In a large bowl, beat the cream cheese until smooth. Gradually mix in granulated sugar and vanilla until combined. Add the eggs one at a time, mixing until just blended, then fold in sour cream and spread over the cooled crust.

In a saucepan, combine diced apples, brown sugar, cinnamon, lemon juice, and a splash of water. Cook over medium heat until softened, about 5-7 minutes. Mix cornstarch with water, stir into the apple mixture to thicken, and let cool.

In a bowl, whisk together flour, brown sugar, cinnamon, and cubed cold unsalted butter until it resembles coarse crumbs.

Layer the apple filling over the cheesecake filling, sprinkle crumble topping generously, and bake in the preheated oven for 55-65 minutes, or until the edges are set.

Turn off the oven, crack the door, and let the cheesecake cool gradually for about an hour. Transfer to the refrigerator and chill for at least 4 hours or overnight.