1. Preheat oven to 350°.
2. Heat olive oil in a large nonstick skillet over medium-high heat. Add onion; cook 2 minutes. Add mushrooms; cook, stirring frequently, 4 minutes or until the mushrooms are tender. Add spinach; cover and cook 1 minute or just until spinach is wilted.
3. In a medium bowl, combine eggs, egg whites, 1/4 cup cheese, salt, and pepper; mix well. Add spinach mixture; mix well. Pour mixture into an 8-inch round cake pan coated with cooking spray. Sprinkle remaining 1/4 cup cheese over frittata. Bake 30 minutes or until eggs are set in center and cheese has melted. Cut into 4 wedges.
Wine note: A velvety, berry-rich California red from the Petite Sirah grape will match the earthiness of the frittata's mushrooms and cut through the rich Gouda flavor. Look for Bogle, a big wine at a relatively small price, about $10.