In a large mixing bowl, gently toss the blueberries with sugar, lemon juice, and flour (or cornstarch) until evenly coated. Transfer this mixture to a baking dish and spread it out evenly.
In a separate bowl, combine the flour, sugar, oats (if using), and cinnamon. Add the cold, cubed butter and cut it into the dry ingredients with a pastry cutter or fork until the mixture resembles coarse crumbs.
Evenly sprinkle the crumbly topping over the blueberry filling, covering all the fruit to create a thick layer.
Place the baking dish in a preheated oven at 375°F (190°C) and bake for about 40 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Remove from the oven and let it cool slightly before serving. The mixture should be bubbling and the topping crisp and flaky, inviting you to scoop out a warm, luscious portion.