Preheat and prepare the pans. Adjust oven rack to middle position and heat oven to 450°. Spray a 9x9 baking dish.
Make the filling. Toss cornstarch, sugar, cinnamon, lemon juice, and berries in a large mixing bowl and pour into baking dish. Wipe out the bowl. 3 tablespoons cornstarch ¼ cup (50 g) granulated sugar ½ teaspoon ground cinnamon 1 teaspoon lemon juice 6 cups (852 g) mixed berries
Make topping. In the same mixing bowl, combine flour, oats, sugar, cinnamon, brown sugar, and salt in bowl. Stir in butter until mixture is thoroughly moistened and crumbly. Sprinkle over the berry mixture. ¾ cups (90 g) all purpose flour ¾ cups (99 g) old fashioned oats ½ cup (107 g) packed light brown sugar ¼ cup (50 g) granulated sugar ½ teaspoon ground cinnamon ½ teaspoon salt 8 tablespoons (1 stick, 113 g) unsalted butter, melted
Bake the crisp. Bake in preheated oven for 15 minutes. Cool slightly before enjoying. Eat with whipped cream,