In a large mixing bowl, gently toss the rinsed blueberries with sugar, lemon zest, and flour until evenly coated and the berries start releasing their juices.
Pour the berry mixture into a baking dish, spreading it out evenly. The filling should look juicy and bubbling at the edges.
In a separate bowl, combine the cold butter, rolled oats, cinnamon, and a pinch of salt. Use a pastry cutter or your fingers to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.
Dollop spoonfuls of the crumble topping over the bubbling berries, covering most of the surface but leaving some gaps for steam to escape.
Place the dish in a preheated oven at 375°F (190°C) and bake for 40 minutes, or until the topping is golden brown and the filling is bubbling vigorously around the edges.
Remove from the oven and let cool slightly — the filling will thicken a bit as it rests, and the aroma of toasted oats and baked berries will fill your kitchen.
