Preheat oven to 350°F.
Stir spinach, mayonnaise, sour cream, garlic and ½ cup Parmesan cheese together in a large bowl. Season with freshly ground black pepper.
Pour into an oven-safe baking dish coated with cooking spray.
Bake in oven for 20 minutes. Sprinkle with remaining ¼ cup Parmesan cheese. Return to oven and bake until cheese melts, about 2 to 3 minutes.
Sprinkle with tomato and green onion before serving.
Kitchen Tip: Serve with cubed French or Italian bread.