Heat 1 tbsp. oil in a large nonstick ovenproof skillet over medium heat.
Add scallions and cook 1 minute, stirring or until softened.
Transfer scallions to a large bowl. Add spinach, eggs, and egg whites, and with a fork until well combined.
Heat remaining 1 tbsp. oil in skillet over medium heat. Pour egg mixture into skillet and scatter the tomatoes on top.
Cover skillet and cook 4 minutes, or until the eggs are set around the edges.
Place skillet in over and broil 4 minutes, or until the frittata is lightly browned and the center is set.
Top with the cheese, cover, and let stand 1 minute while cheese melts.
Cut frittata in 4 and serve each with 1 slice toast.