To make this Cheesy Cabbage Casserole Recipe gluten free, replace the all-purpose flour with a certified gluten-free all-purpose flour blend. Verify that the cornstarch, baking powder, ricotta cheese, sour cream, mozzarella, cheddar, olive oil, seasonings, and cooking spray are gluten free. Use clean cookware, utensils, and prep surfaces to avoid cross-contact with gluten-containing ingredients.