Ina&#39;s Best Zucchini Gratin features a base of tender zucchini and onions and a cheesy breadcrumb topping.  #RecipeOfTheDay: Zucchini Casseroles, Contessa Zucchini, Side Dishes, Zucchini Gratin, Casseroles Recipe, Barefoot Contessa, Ina Garten, Breads Crumb, Buttons Recipe