Heat the oven. Preheat to 375°F (190°C). Lightly butter a 9x9-inch baking dish or a 2-quart casserole.

Toss the fruit. In the baking dish, combine the frozen berries, granulated sugar, cornstarch, lemon zest, lemon juice, and vanilla. Stir gently to coat. Spread the fruit into an even layer. Do not thaw the berries.

Mix the crumble topping. In a bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add the cold butter cubes. Use your fingers or a pastry cutter to work the butter into the dry mix until it looks like coarse crumbs with some pea-sized bits. Stir in nuts if using.

Top the fruit. Sprinkle the crumble evenly over the berries. If you like extra crunch, dust the top with turbinado sugar.

Bake. Place the dish on a rimmed baking sheet to catch any drips. Bake for 40–50 minutes, until the topping is golden brown and the fruit is bubbling thickly at the edges. If the top browns too fast, tent loosely with foil.

Rest before serving. Let the crumble sit for 10–15 minutes. This helps the juices thicken so each scoop holds together.

Serve. Enjoy warm with vanilla ice cream, whipped cream, or Greek yogurt.