Begin by preheating your oven to 350°F. Prepare two standard muffin pans by lining them with 18 parchment cupcake or muffin liners, as they peel away more easily than traditional paper ones for this dessert.
In a medium mixing bowl, combine the graham cracker crumbs, melted unsalted butter, and salt, stirring until the mixture is well blended and the crumbs adhere together.
Evenly press 1½ tablespoons of the graham cracker crumb mixture into the bottoms of each muffin cup. Set the pans aside for later use.
In a large mixing bowl, use a handheld mixer set to medium speed to beat the cream cheese and granulated sugar for about 1 to 2 minutes, or until the mixture is smooth. Be sure to scrape down the sides and bottom of the bowl to fully incorporate all the ingredients.
Add the eggs one by one, mixing just until each is combined with the cream cheese mixture. Avoid overmixing to keep excess air from entering your cheesecake filling.
Gently fold in the sour cream, all-purpose flour, and vanilla extract until everything is smooth and well combined.
Distribute 2 tablespoons of the cheesecake filling into each muffin cup lined with graham cracker crumbs.
Next, place your apple pie filling onto a large cutting board and roughly chop any larger apple pieces. This step isn’t necessary if your brand contains small chunks rather than full slices. Once chopped, transfer the apple pie filling to a bowl for easier scooping to fill the muffin cups.
Spoon 1 to 1½ tablespoons of the chopped apple pie filling into the center of each cheesecake-filled muffin cup. Set the cups aside for later.
To prepare the streusel topping, combine the all-purpose flour, granulated sugar, light brown sugar, finely chopped walnuts, baking powder, cinnamon, and salt in a medium mixing bowl. Stir the ingredients together until well mixed.
Drizzle the melted butter over the dry mixture and gently stir until the butter is mostly absorbed and large crumbs begin to form.
Next, add the heavy cream and mix to ensure that all the dry ingredients are incorporated, resulting in a crumble mixture with both small and large crumbs.
Sprinkle 1½ to 2 tablespoons of the streusel topping over each mini apple cheesecake.
Bake in the preheated oven for 20 minutes, or until the streusel is a light golden brown and the cheesecake has set.
Once baked, let the mini apple cheesecakes cool in the pan on the counter for 30 minutes. Then, transfer the pan to the refrigerator and chill for an additional 2-3 hours, or until the cheesecake layer is fully set and cooled.