Preheat and prep: Heat your oven to 375°F (190°C). Lightly butter a 9x9-inch baking dish or a similar 2-quart dish.

Mix the fruit: In a large bowl, combine berries, granulated sugar, lemon zest, lemon juice, cornstarch, and vanilla. Gently toss to coat without crushing the berries.

Make the crumble: In another bowl, whisk flour, oats, brown sugar, cinnamon, and salt. Add the cold butter cubes. Use your fingertips or a pastry cutter to rub the butter into the dry mix until you get pea-sized clumps and a sandy texture.

Assemble: Spread the berry mixture evenly in the baking dish. Sprinkle the crumble topping over the fruit, covering it completely but leaving some craggy bits for texture.

Bake: Place the dish on a rimmed baking sheet to catch any drips. Bake for 35–45 minutes, until the topping is golden brown and the fruit is bubbling thickly around the edges.

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

Serve: Spoon into bowls and top with ice cream, whipped cream, or yogurt. Enjoy warm.