Preheat your oven to 160ºC (325ºF). In a food processor, blend the digestive or graham crackers with sugar until they resemble fine crumbs. Combine with melted butter, stirring until well mixed. Press this mixture firmly into the bottom of a springform pan and bake for 10 minutes, allowing it to set before letting it cool completely.

In a medium bowl, gently mix fresh blueberries with sugar, flour, and lemon juice until they are well-coated. For the crumble, combine the remaining flour, brown sugar, and melted butter in a separate bowl, mixing until it achieves a crumbly texture. Set both the blueberry mixture and crumble aside while you prepare the cheesecake filling.

In a large mixing bowl, beat the full-fat cream cheese and granulated sugar using a hand mixer until smooth and creamy, about 3-4 minutes. Next, add sour cream blended with cornstarch and vanilla extract, mixing until combined. Gradually beat in large eggs, one at a time, ensuring the filling is silky and well incorporated but not overmixed to maintain its texture.

Pour the cheesecake filling evenly into your cooled crust, spreading it out gently. Top the filling with the blueberry mixture, distributing the blueberries for even coverage. Finally, sprinkle the crumble topping generously over the blueberries.

Prepare a water bath by placing a larger baking pan filled with hot water on the lower rack of the oven. Place the springform pan with the assembled cheesecake on the middle rack. Bake for 1 hour and 20-30 minutes until the center is slightly wobbly but set.

After baking, turn off the oven, crack the door open, and let the cheesecake cool inside for 1 hour. Carefully transfer it to a cooling rack to cool completely for another hour. Once cooled, refrigerate your cheesecake for at least 6 hours or preferably overnight.