Slow Cooker Berry Cobbler

Name: Slow Cooker Berry Cobbler
Ingredients: Unsalted butter, for greasing insert; 1/4 cup cornstarch; 2 tablespoons lemon juice (about 1 lemon); Two 12-ounce bags frozen mixed berries (about 5 1/2 cups); 2/3 cup granulated sugar; 1 1/4 cups all-purpose flour; 3 tablespoons granulated sugar; 1 teaspoon baking powder; Pinch fine salt; 4 tablespoons cold unsalted butter, cut into small pieces; 3/4 cup milk; 1/4 teaspoon ground cinnamon; 1/2 cup heavy cream; 1/4 cup sour cream; 2 tablespoons confectioners' sugar