Prep: 8 minutes; Cook: 6 minutes
1. Preheat broiler.
2. Heat oil in a 10-inch ovenproof skillet over medium heat. While oil heats, coarsely chop spinach and finely chop onions. Combine egg whites, eggs, cheese, Greek seasoning, and salt in a large bowl; stir well with a whisk. Add spinach and onions, stirring well.
3. Add egg mixture to pan; cook until edges begin to set, about 2 minutes. Gently lift edge of egg mixture, tilting pan to allow uncooked egg mixture to come in contact with pan. Cook 2 minutes or until egg mixture is almost set.
4. Broil 2 to 3 minutes or until center is set. Transfer the frittata to a serving platter immediately; cut into 4 wedges.