1. Preheat your oven to 375 F (190 C). Lightly grease an 8×8 inch or 9×9 inch baking dish with butter or non-stick spray. This prevents the blueberry filling from sticking and makes for easier serving.

2. In a large bowl, combine the fresh or frozen blueberries, granulated sugar, 2 tablespoons of all-purpose flour, lemon juice, and ground cinnamon. Stir gently until the blueberries are evenly coated. The flour helps to thicken the juices as the crisp bakes, creating a luscious filling.

3. Pour the blueberry mixture into your prepared baking dish, spreading it out into an even layer. This ensures consistent cooking and a balanced ratio of fruit to topping in every bite.

4. In a separate medium bowl, combine the old-fashioned rolled oats, 1/2 cup all-purpose flour, light brown sugar, and salt. Mix these dry ingredients together thoroughly.

5. Add the cold, cubed butter to the dry oat mixture. Using your fingertips, a pastry blender, or two knives, cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces remaining. Avoid overworking the mixture; the goal is a crumbly texture, not a smooth dough.

6. Sprinkle the oat crumble topping evenly over the blueberry filling in the baking dish. Place the dish on a baking sheet (to catch any potential drips) and bake for 35-40 minutes, or until the blueberry filling is bubbly around the edges and the oat topping is golden brown and crisp.

7. Once baked, remove the crisp from the oven and let it cool for at least 10-15 minutes before serving. This allows the filling to set slightly, preventing it from being too runny, and ensures the topping remains crisp.