In a bowl, combine blueberries, flour, sugar (if using), lemon juice, and lemon zest.
Spread blueberry mixture in the bottom of a small baking dish. (I used an 8x8 glass pan.)
In a separate bowl, combine the remaining ingredients and layer over the blueberry mixture.
Bake on the center oven rack for 30-40 minutes, until the granola topping is golden brown.
Allow to cool at least 5 minutes.
Serve with nice cream or your favorite vanilla ice cream****, if desired.
Store any leftovers covered in the refrigerator for 3-4 days.
Notes
* Use however many blueberries you have. If you only have 4 cups, that works; but if you have more, that makes this crumble even better.** You can use any kind of flour, including gluten-free. Flour is used to absorb and thicken some of the natural juices resulting from baking the blueberries. Use less if you want more juice or sauce. Also adjust depending on how many blueberries you have for this recipe.*** You can use any chopped nuts you prefer. Walnuts and almonds are delicious on top of this crumble.**** I highly recommend serving this with your favorite vanilla ice cream. I have served with frozen banana nice cream as well as Aldi’s vanilla plant-based ice cream. Both are delicious.