Preheat and prep: Heat the oven to 375°F (190°C). Lightly butter a 9-inch square baking dish or similar 2-quart dish.

Mix the filling: In a large bowl, combine blueberries, lemon zest, lemon juice, granulated sugar, cornstarch, vanilla, cinnamon, and a pinch of salt. Toss until the berries are evenly coated and glossy. If using frozen berries, use the full 3 tablespoons cornstarch.

Make the crumble topping: In another bowl, whisk flour, oats, brown sugar, and 1/4 teaspoon salt. Cut the cold butter into small cubes and work it into the dry mix using a pastry cutter or your fingertips until it forms pebble-sized clumps. If using nuts, fold them in.

Assemble: Spread the blueberry mixture in the prepared dish. Sprinkle the crumble topping evenly over the berries, leaving some larger clumps for extra crunch.

Bake: Place the dish on a baking sheet to catch drips. Bake 35–45 minutes, until the topping is golden and the berries are bubbling around the edges. If the top browns too fast, tent loosely with foil.

Rest: Let the crumble cool for at least 15–20 minutes. This helps the juices thicken so the filling isn’t runny.

Serve: Spoon into bowls while warm. Add a scoop of vanilla ice cream or a dollop of Greek yogurt for a creamy finish.