1. 1. Prepare Your Oven and Baking Dish: Preheat your oven to 375 F (190 C). Lightly grease an 8×8 inch or 9×9 inch baking dish with butter or cooking spray. This prevents sticking and makes serving easier.

2. 2. Mix the Blueberry Filling: In a large bowl, combine the blueberries, 1/2 cup granulated sugar, and 1/4 cup all-purpose flour. If using fresh blueberries, you can also add the optional 1 tablespoon of lemon juice for a brighter flavor. Gently toss until the blueberries are evenly coated. Pour the blueberry mixture into your prepared baking dish, spreading it out into an even layer.

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

4. 4. Cut in the Butter for the Topping: Add the cold, cubed butter to the dry topping mixture. Using your fingertips, a pastry blender, or two forks, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. This creates that wonderful crumbly texture.

5. 5. Assemble the Crisp: Evenly sprinkle the oat topping mixture over the blueberry filling in the baking dish. Make sure the blueberries are mostly covered.

6. 6. Bake Until Golden and Bubbly: Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the topping is golden brown and the blueberry filling is bubbly around the edges. You should see the fruit juices thickening.

7. 7. Cool Slightly Before Serving: Remove the crisp from the oven and let it cool on a wire rack for at least 10-15 minutes before serving. This allows the filling to set slightly and prevents it from being too runny.