In a mixing bowl, combine the thawed and drained blueberries with granulated sugar and lemon juice. Toss gently until the berries are coated and the juices start to ooze out, then transfer the mixture to a baking dish.
In a separate bowl, whisk together the flour, sugar, and cinnamon (if using). Add the cold, cubed butter and use a pastry cutter or fork to rub the butter into the flour mixture until it resembles coarse crumbs with pea-sized pieces.
Sprinkle the crumbly topping evenly over the blueberry filling, covering all the berries. Gently press down the topping to help it adhere and create a nice crust.
Place the baking dish in a preheated oven at 375°F (190°C) and bake for about 35 minutes, or until the topping is golden brown and the filling is bubbling around the edges. The aroma of baked fruit, butter, and spices will fill your kitchen.
Remove from the oven and let it cool slightly for about 10 minutes. The filling will thicken as it cools, and the topping will become crisp and flaky.
Serve warm, optionally with a scoop of vanilla ice cream or a dollop of whipped cream for extra indulgence. Enjoy the sweet, juicy blueberries topped with a buttery, crumbly crust.
