Preheat the oven to 350°F (177°C).
To prepare the filling, whisk together sugar, cornstarch and salt in small bowl.
¼ cup granulated sugar, 1 tablespoon cornstarch, ½ cup all-purpose flour, ¼ teaspoon coarse kosher salt
Place the fruits in 9-inch baking dish.
4 cups mixed fruits
Add sugar mixture over fruits and mix everything.
To prepare the topping, combine the dry ingredients in a medium bowl.
1 cup old-fashioned rolled oats, ½ cup packed brown sugar, ¼ teaspoon coarse kosher salt
Add the melted butter and mix until all the dry ingredients are evenly moistened.
1/3 cup unsalted butter
Spread the topping evenly over the fruits.
Bake the crisp for 40-45 minutes, or until the crust is nice and golden.
Serve warm or at room temperature with a vanilla ice cream, if desired.