Heat oven to 350°F 175°C and grease an 8 inch square or similar baking dish.
In a bowl toss blueberries with granulated sugar and lemon juice then spread in the dish.
In another bowl stir flour oats brown sugar cinnamon and salt.
Pour in the melted butter and mix with a fork until large crumbs form.
Scatter the crumbs over the berries.
Bake 35 to 40 minutes until the top is golden and the filling bubbles at the edges.
Cool 10 minutes so the juices settle then serve warm or at room temperature.