Preheat and prep: Heat your oven to 350°F (175°C). Lightly butter a 9-inch square baking dish or a similar 2-quart dish.

Mix the fruit filling: In a large bowl, combine blueberries, granulated sugar, lemon juice, lemon zest, vanilla, cornstarch, and a pinch of salt. Gently toss until the berries are evenly coated. Transfer to the baking dish and spread into an even layer.

Make the crumble topping: In a separate bowl, stir together oats, flour, brown sugar, cinnamon, and a pinch of salt. Pour in the melted butter and mix with a fork until the mixture forms moist, crumbly clumps. If using nuts, fold them in now.

Top the fruit: Sprinkle the crumble evenly over the blueberries, leaving a few gaps so steam can escape. Aim for a mixture of pea-sized and larger clumps for the best texture.

Bake: Place the dish on a sheet pan to catch any bubbling juices. Bake for 35–45 minutes, until the topping is golden and the berries are thick and bubbling around the edges.

Rest before serving: Let the crumble sit for 15–20 minutes. This helps the juices thicken so each scoop holds together.

Serve: Enjoy warm with ice cream, whipped cream, or a spoonful of Greek yogurt. It’s also great at room temperature.