Preheat the oven to 350℉. While the oven heats, rinse and clean berries under cool water Gently pat dry, pour into a mixing bowl and set aside.
In a separate bowl, mix together all of the dry ingredients for the berry filling. Stir until combined. Sprinkle over the bowl of berries, add lemon juice, and stir gently until the berries are evenly coated. Cover the bowl of berries with plastic wrap and place it in the fridge to keep cool while you prepare the crumb topping.
To make the topping, add all of the remaining dry ingredients into a bowl, including the oats, brown sugar, flour, cinnamon, and salt. Cut in the cold, cubed butter with a fork or pastry cutter until the mixture looks like pea-sized crumbs.
Remove the bowl of berries from the fridge and gently spread them on the cast iron skillet. Sprinkle the crumb topping over the berries and place the skillet in the oven.
Bake for 30-35 minutes or until the fruit is bubbling and the crisp topping is golden brown.
Let cool for one hour, or until the fruit filling has set.
Serve with vanilla ice cream or whipped cream (if desired) and garnish with mint.
Notes
Leftovers should be stored in an airtight food storage container and can be kept in the fridge for up to four days.