Preheat oven to 400 degrees. Grease a 13-inch roasting pan. Rub chicken with 1/4 cup marinade. In a mixing bowl toss onions and zucchini with remaining 1/4 cup marinade and spread them in bottom of prepared pan. Place chicken, breast-side up, on top of vegetables and cook 30 minutes. Turn chicken over and bake 30 minutes more. Turn chicken breast-side up and cook until golden brown and thermometer inserted in thigh registers 180 degrees, about 20 minutes longer. Let sit 10 minutes before carving.