1. Prepare the Crust:

1. Preheat your oven to 325°F (163°C).
2. In a medium-sized mixing bowl, combine the graham cracker crumbs and granulated sugar.
3. Stir in the melted butter and mix until the crumbs are fully coated and the texture resembles wet sand.
4. Press the mixture evenly into the bottom of a 9-inch springform pan. Make sure to press it down firmly.
5. Bake for 10 minutes, then remove from the oven and set aside to cool. This step sets the crust and helps it stay firm during the cheesecake assembly.

2. Make the Blueberry Filling:

1. In a medium saucepan, combine fresh blueberries, sugar, lemon juice, and cornstarch.
2. Cook over medium heat, stirring occasionally, until the blueberries release their juices and the mixture thickens. This should take about 5–7 minutes.
3. Once thickened, remove from heat and set aside to cool to room temperature.

3. Prepare the Cheesecake Filling:

1. In a large mixing bowl, beat the softened cream cheese with an electric mixer until smooth and creamy.
2. Add the sugar and vanilla extract, and continue beating until fully incorporated.
3. Add the eggs one at a time, beating well after each addition.
4. Stir in the sour cream until the mixture is light, smooth, and creamy.

4. Assemble the Cheesecake:

1. Pour half of the cheesecake filling over the prepared crust.
2. Spoon half of the blueberry filling evenly over the cheesecake layer.
3. Repeat with the remaining cheesecake filling and blueberry filling, creating a marbled effect by swirling the blueberry filling through the cheesecake with a knife.
4. Set the cheesecake aside while you prepare the crumble topping.

5. Prepare the Crumble Topping:

1. In a small bowl, combine the flour, oats, and brown sugar.
2. Add the cold butter and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
3. Sprinkle the crumble topping evenly over the cheesecake.

6. Bake the Cheesecake:

1. Place the springform pan on a baking sheet to catch any potential drips.
2. Bake for 50–60 minutes, or until the center of the cheesecake is set and the top is golden brown.
3. Once done, turn off the oven and leave the cheesecake inside with the oven door slightly ajar for 1 hour. This cooling step helps prevent cracks from forming.
4. After 1 hour, remove the cheesecake from the oven and allow it to cool to room temperature.
5. Refrigerate for at least 4 hours, or preferably overnight, to allow the cheesecake to fully set.

7. Serve:

1. Once the cheesecake has chilled and set, carefully run a knife around the edge of the pan to loosen it, then remove the sides of the springform pan.
2. Slice the cheesecake into wedges and serve chilled. Optionally, garnish with additional fresh blueberries or a dollop of whipped cream.