Combine the blueberries, sugar, lemon juice, and 2 tablespoons of flour in a large bowl. Stir gently until the berries are coated and start releasing juice. Let sit for 10 minutes as the mixture thickens and bubbles slightly.
Pour the blueberry filling into a buttered baking dish, spreading it out evenly. The filling should be bubbling at the edges and thickened in the center.
In a separate bowl, whisk together the 1 1/2 cups of flour, sugar, oats if using, and cinnamon. Add the cold butter cubes and cut in with a pastry cutter or fork until the mixture resembles coarse crumbs.
Sprinkle the crumb topping evenly over the blueberry filling, covering all the fruit but leaving some gaps for the filling to bubble through.
Bake in a preheated oven at 375°F (190°C) for 40 minutes, until the topping is golden brown and crisp, and the filling is bubbling around the edges.
Remove from the oven and let cool slightly before serving. The bubbling filling and crunchy topping make a perfect contrast, inviting you to scoop and enjoy.