Preheat oven to 350ºF. Combine spinach, mayonnaise, sour cream, 1/2 cup Parmesan cheese and garlic in large bowl. Season, if desired, with freshly ground black pepper. Turn into 2-quart baking dish sprayed with no-stick cooking spray.
Bake 20 minutes. Sprinkle with remaining 1/4 cup cheese, then continue baking until cheese is melted, about 2 minutes.
Sprinkle with tomato and green onion. Serve, if desired, with cubed French or Italian bread.