If you'd like to cook the sauce completely from scratch, substitute a large can of crushed tomatoes for the premade sauce. You'll also want to add some garlic and spices (like basil, oregano, crushed red pepper flakes, etc.) to flavor the sauce. Add the garlic to the cooked yellow squash, then stir constantly for about a minute or so, or until the garlic is fragrant. Stir in the crushed tomatoes and spices, then simmer until flavors blend. Finish the recipe as directed.