Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish (or a 2.5-quart casserole dish) with butter or cooking spray.

In a large mixing bowl, gently combine the blueberries, granulated sugar, 2 tablespoons of all-purpose flour, lemon juice, and vanilla extract. Stir just until the blueberries are evenly coated. Pour the berry mixture into your prepared baking dish and spread it into an even layer.

In a separate medium mixing bowl, whisk together the 1 cup of all-purpose flour, rolled oats, brown sugar, cinnamon, and salt. Add the cold, cubed butter to the dry ingredients. Using your fingers, a pastry blender, or a fork, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.

Evenly sprinkle the oat topping mixture over the blueberry filling in the baking dish. Don't press it down; you want it to be light and crumbly.

Bake in the preheated oven for 35-40 minutes, or until the filling is bubbly around the edges and the topping is golden brown and crisp. If the topping starts to get too dark, you can loosely tent it with foil.

Remove the crisp from the oven and let it cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set up a bit. Serve warm.