Preheat oven to 375 degrees F. In a large bowl, stir together the 3 tablespoons flour and the granulated sugar. Add blueberries and lemon juice; toss gently to combine. Spread berry mixture evenly in an ungreased 3-quart rectangular baking dish; set aside.
For topping, in a medium bowl, combine brown sugar, the 3/4 cup flour, the oats, and cinnamon. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Sprinkle topping evenly over berry mixture.
Bake, uncovered, about 30 minutes or until topping is golden brown and edges are bubbly. Cool on a wire rack about 45 minutes. Serve warm with vanilla ice cream, if desired.