Preheat oven to 375 degrees. Make the filling: Mix blueberries, sugar, cornstarch, lemon juice, and salt in a bowl. Transfer to an 8-inch square baking dish.
Make the topping: Stir together flour, oats, baking powder, and salt. Cream butter and sugar in a mixer until pale and fluffy. Stir dry ingredients into butter. Using your hands, squeeze topping pieces together into clumps.
Sprinkle topping evenly over filling. Bake until bubbling in center and brown on top, about 1 hour. Transfer to a wire rack, and let cool for 30 minutes before serving.
Make and freeze extra batches of the topping. Whenever you have fresh fruit on hand, you'll be able to assemble the dessert quickly.