Preheat oven to 350℉. Pour the frozen berries (unthawed) in a 9x13 baking dish. Add the cornstarch, sugar, and lemon zest. Toss well to combine.
Prepare the topping. In a food processor, pulse the flour, brown sugar, butter, cinnamon, and salt until mixture turns into crumbs. You can also do this by hand by cutting in the butter with a pastry cutter or two forks. Add the oats and pulse barely to combine.
Sprinkle mixture on top of the berries. Bake 40-45 minutes or until mixture is brown. Let crumble cool for about 10 minutes. This allows the mixture to thicken up. Serve warm with ice cream. Best if eaten soon after baking as the crumble will soften over time.