In a large mixing bowl, gently toss the blueberries with sugar, lemon zest, lemon juice, and cinnamon until evenly coated. Transfer this mixture into a sturdy baking dish, spreading it out into an even layer. The berries should be juicy and fragrant.
In a separate bowl, combine the flour, oats (if using), and brown sugar. Add the cold butter pieces, then use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs. This creates a flaky, crumbly topping.
Sprinkle the crumb topping evenly over the blueberry filling, covering all the fruit but leaving some gaps for steam to escape. Lightly press down on the topping to help it adhere to the fruit underneath.
Place the baking dish in a preheated oven at 375°F (190°C). Bake until the topping is golden brown and bubbling around the edges, about 35 minutes. You’ll smell the toasty aroma of baked fruit and butter wafting through the kitchen.
Remove the cobbler from the oven and let it cool slightly for 10 minutes. The filling will thicken a bit as it cools, and the topping will set up with a crisp, buttery texture. Serve warm, with a scoop of vanilla ice cream if desired.
