Preheat your oven to 375°F (190°C). Grease a baking dish with butter or non-stick spray.
In a mixing bowl, gently toss the blueberries with sugar, lemon zest, flour, and lemon juice until evenly coated. This helps create a juicy, flavorful filling.
Pour the blueberry mixture into the prepared baking dish, spreading it evenly with a spatula or spoon.
In a separate bowl, combine flour, oats, brown sugar, cinnamon, and cold butter. Use a pastry cutter or fork to cut in the butter until the mixture resembles coarse crumbs.
Sprinkle the crumble topping evenly over the blueberry filling, covering all the fruit but leaving some uneven patches for rustic charm.
Bake in the preheated oven for 35-40 minutes, or until the topping is golden brown and the filling is bubbling around the edges. You’ll hear a satisfying crackle when gently shaking the dish.
Remove from the oven and let cool slightly for 10 minutes. Serve warm, ideally with a scoop of vanilla ice cream or a dollop of whipped cream.