Preheat oven to 400°F. Butter a 10-in cast-iron skillet.

Combine berries, sugar, cornstarch, salt, and lemon juice in large bowl. Mix until all berries coated and set aside.

Combine flour, oats, brown sugar, cinnamon, and salt in med bowl and whisk. Work butter into flour mixture with hands until mixture resembles coarse meal.

Pour berry mixture into skillet. Evenly sprinkle topping over berries. Bake until topping is golden brown and berries are bubbling. 30 min.

Let sit 5-10 min before serving. Store in individual mason jars.