Preheat oven to 375°F. Grease a 6 or 8 cup baking dish and set aside.
In a large bowl (or directly in baking dish), mix together blueberries, flour, sugar, lemon juice, and lemon zest. Place in baking dish.
To make the topping, using a pastry cutter, mix together the oats, butter, sugar, flour, cinnamon, and salt if using. Once fully mixed, use a spoon to stir in nuts if using. Spoon topping on top of fruit.
Place baking dish on a baking tray (line with parchment in case of any spills).
Bake 35-40 minutes, until bubbling on the edges. Check if done by using a fork or knife in the center. If it starts to brown but the fruit is not ready, cover loosely with foil or parchment and bake for 5 more minutes. If still not ready, check after 3 minutes until it is cooked to your liking.
Let cool for 5 minutes before serving to thicken the fruit mixture.
Notes
You may use 5 cups of any fruit mixture. For rhubarb, increase sugar to ⅔ cup sugar, and increase flour to 3 Tablespoons. Smart tip: Double the topping and put half in a Ziploc bag in your fridge (or freezer). It will keep for a couple of weeks. Simply add on top of your next fruit mixture and bake as directed!