Preheat the oven to 375°F.
Grate the rind of the lemon and squeeze the juice of half of the lemon.
Toss blueberries, with sugar, lemon rind, 1 tablespoon lemon juice and 3 tablespoons flour. Place in a 2qt baking dish.
With a fork, combine the butter, brown sugar, oats, flour, nuts if using and cinnamon until crumbled. Sprinkle over blueberries.
Bake 35-40 minutes or until the top is golden brown and the fruit is bubbly. Cool slightly and serve warm.