Heat olive oil in a large ovenproof nonstick skillet over medium heat. Cook scallions for 1 minute, stirring, or until softened.
Transfer scallions to a large bowl and set aside.
Add spinach, whole eggs, and egg whites to the bowl with the scallions. Beat with a fork until well blended.
Heat the remaining oil in the skillet over medium heat. Pour in egg mixture and scatter tomatoes on top. Cover skillet and cook for 4 minutes, or until eggs are set around edges.
Broil 5 inches from the heating element. Broil for 4 minutes, or until the frittata is lightly browned and the center is set. Top with cheese, cover, and let stand for 1 minute to melt cheese. Cut into 4 wedges and serve.
