1. Preheat your oven to 375 F (190 C). Lightly grease a 9×13-inch baking dish with butter or cooking spray.

2. In a large bowl, combine the fresh or frozen blueberries, 1/2 cup granulated sugar, 2 tablespoons all-purpose flour, lemon juice, and vanilla extract. Toss gently until the blueberries are evenly coated. Pour this blueberry mixture into your prepared baking dish, spreading it out evenly.

3. In a separate medium bowl, whisk together the old-fashioned rolled oats, 1/2 cup all-purpose flour, brown sugar, ground cinnamon, and salt. Make sure there are no lumps of brown sugar and all ingredients are well combined.

4. Add the cold, cubed unsalted butter to the dry topping mixture. Using your fingertips, a pastry blender, or two knives, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

5. Evenly sprinkle the oat topping mixture over the blueberry filling in the baking dish. Ensure the topping covers the blueberries from edge to edge for consistent crispness.

6. Place the baking dish in the preheated oven and bake for 35-40 minutes. The crisp is ready when the topping is golden brown and the blueberry filling is bubbly around the edges.

7. Once baked, remove the crisp from the oven. Let it cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set slightly, making it easier to scoop and preventing it from being too runny.