1. Preheat broiler to high. Combine egg whites, eggs, salt, pepper, and milk; stir with a whisk.
2. Heat a 9-inch cast-iron skillet over medium heat. Sauté mushrooms and leeks in olive oil until tender.
3. Layer in thyme and green onions.
4. Sprinkle with shredded fontina cheese.
5. Carefully pour egg mixture over cheese. Cook over medium heat 5 to 6 minutes or until eggs are partially set; then broil 5 inches from heat for 2 to 3 minutes or until browned and almost set.