Preheat the oven to 375°F / 190°C (170° fan-assisted).
Put the blueberries in a bowl and add the lemon juice along with 2 tablespoons of the flour and 3 tablespoons of the sugar. Toss to coat evenly and set aside.
In a separate bowl, combine the remaining flour and sugar with the salt and stir together.
Pour the melted butter into the dry ingredients and use a fork to mix it through. Keep mixing until it is evenly distributed and you have thick, moist clumps of crumble.
Put the coated blueberries into your baking dish in an even layer.
Use your fingers to 'sprinkle' the crumble topping all over the blueberries, covering them completely.
Bake for 35-40 minutes, until the juices are thick and the crumble is lightly golden brown on top.
Remove from the oven and let cool for 10-15 minutes before serving.
Notes
This recipe is developed for an 8 x 8 baking dish or equivalent. For a 9 x 13 inch pan, double the recipe. Frozen Blueberries - I recommend using fresh blueberries, but you can use frozen blueberries if it's not blueberry season. Thaw the blueberries completely and drain off excess liquid before using and add an extra tablespoon of flour and sugar.