Preheat the oven to 375°F (190°C).
Add the blueberries, lemon juice, 2 tablespoons of flour, 2 tablespoons of sugar, and a pinch of salt to a 9-inch pie dish (or use a 2-quart baking dish). Toss well until combined.
In a medium bowl, combine the flour, rolled oats, sugar, and salt in a medium bowl. Add the melted butter, then stir until the butter has moistened all the flour and the topping is crumbly.
Sprinkle the crumble topping over the blueberry filling. I use my hands to do this. As I sprinkle, I press the crumbles together to create larger clumps of topping.
Bake until the juices thicken, and the topping turns light golden brown, 25 to 35 minutes. Cool for 10 minutes, and then serve.
