Preheat the oven 400 degrees F. and spray an 8x8 inch square baking dish or 10 inch cast iron skillet with non-stick cooking spray or use butter.
Using a large bowl, combine the 6 cups of mixed berries, ½ cup of sugar, 2 tablespoons cornstarch, ¼ teaspoon salt, lemon juice, and gently stir until all of the berries are fully coated and pour into the prepared dish or skillet and set aside.
In a separate bowl, prepare the crisp topping by whisking together the ¾ cup all-purpose flour, ¾ cup oats, ½ cup brown sugar, ½ teaspoon cinnamon, and ½ teaspoon salt.
Next work the 6 tablespoons butter into the mixture using your hands until it resembles a coarse meal and all of the butter is distributed throughout the mixture.
Pour the mixture over top of the berries in the dish or skillet making sure the topping is covering all of the berries on top.
Place into the oven and bake about 30 minutes until the crisp layer is golden brown. Allow cooling in the pan for 10 minutes prior to serving. Serve warm topped with your favorite ice cream, the sauce with thicken as it cools. Store any leftovers in the refrigerator up to 3 days.