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 spinach and minced garlic in olive oil until tender.
3. Layer in prosciutto, caramelized onions, and chives.
4. Sprinkle with shredded Gruyere 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.