Preheat your oven to 375°F (190°C). In a large bowl, gently toss the blueberries with sugar and lemon juice until well coated. Transfer the berry mixture to a baking dish and spread evenly.
In a separate bowl, combine the flour, cinnamon (if using), and oats. Add the cold butter pieces, and use a pastry cutter or fork to blend until the mixture resembles coarse crumbs, with some larger clumps for texture.
Sprinkle the crumble mixture evenly over the blueberries in the baking dish, covering the fruit completely but loosely, allowing some of the juice to peek through.
Place the dish in the preheated oven and bake for about 35 minutes, or until the topping turns golden brown and the filling is bubbling around the edges with thickened juices.
Remove the cobbler from the oven and let it cool for a few minutes. The aroma of toasted oats and baked berries should fill your kitchen. Serve warm, ideally with a scoop of ice cream or a dollop of whipped cream for extra indulgence.