Preheat oven to 450°.
Toss onions with 2 tablespoons Wish-Bone® Italian Dressing on broiler pan without rack. Add mushrooms in single layer. Roast 25 minutes or until vegetables are tender. Slice mushrooms and toss with 2 tablespoons Dressing; set aside.
Just before serving, toss spinach with remaining 1/4 cup Dressing. Arrange spinach on serving platter, then top with mushrooms, onions and chicken. Sprinkle, if desired, with grated Parmesan cheese.