Preheat your oven to 160ºC (325ºF). In a food processor, blend digestive cookies or graham crackers with sugar until finely crushed. Combine with melted unsalted butter until fully mixed. Press this mixture into the bottom of a springform pan. Bake for 10 minutes until slightly golden, then cool completely.

In a mixing bowl, combine fresh blueberries with granulated sugar, flour, and lemon juice. Stir gently until blueberries are fully coated. Set aside.

In a separate bowl, mix all-purpose flour with dark brown sugar. Add melted butter and stir until the mixture resembles coarse crumbs. Set aside.

Beat full-fat cream cheese with an electric mixer on medium speed until smooth. Gradually add granulated sugar, then sour cream, cornstarch, and vanilla extract. Add eggs two at a time, mixing until combined but not overbeaten.

Pour the cheesecake batter over the cooled crust, smoothing it out. Distribute the blueberry filling evenly on top. Sprinkle the crumble topping over the blueberries.

Place the springform pan into a larger baking dish filled with hot water. Bake for 1 hour and 20–30 minutes until the center is wobbly but set. The edges should be firm and the top slightly golden.

Turn off the oven and crack the door open, letting the cheesecake cool for about 1 hour. Remove the pan from the water bath and cool to room temperature before refrigerating for at least 6 hours or overnight.