Summer Berry Crisp

Name: Summer Berry Crisp
Cuisine: American
Ingredients: 2 tablespoons unsalted butter, cut into cubes, plus 1 tablespoon for the pan; 1 pound fresh or frozen IQF blackberries (no need to thaw any of the berries if using frozen); 1 pound fresh or frozen IQF blueberries; 1 pound fresh or frozen IQF strawberries; 3⁄4 cup granulated sugar; 1⁄3 cup all-purpose flour; 2 teaspoons vanilla extract; Brown Sugar Oatmeal Topping; Whipped cream or vanilla ice cream, for serving (optional)