Special equipment: a 10-inch cast-iron skillet
Preheat the oven 400 degrees F. Butter a 10-inch cast-iron skillet.
Combine the berries, granulated sugar, cornstarch, salt and lemon juice in a large bowl. Mix until all the berries are coated. Set aside.
For the topping: Combine the flour, oats, brown sugar, cinnamon and salt in a medium bowl, whisking to combine. Work the butter into the flour mixture with your hands, pressing the butter and flour mixture between your fingers until the mixture resembles a coarse meal.
Pour the berry mixture into the prepared skillet. Evenly sprinkle the topping mixture over the berries. Bake until the topping is golden brown and the berry mixture is bubbling, about 30 minutes. Let sit for 5 to 10 minutes before serving.