Preheat your broiler.
Combine all of the ingredients except 1/4 cup of the mozzarella and tomatoes. Add salt and pepper to your taste.
Heat oil in a broiler safe skillet on medium.
Pour egg mixture into skillet and arrange tomatoes on top. Cover and cook 5 to 6 minutes, or until it is just set around the edges.
Place skillet in broiler and broil frittata 4 to 5 minutes.
Sprinkle with remaining mozzarella and broil until cheese melts.
Losen frittata from skillet and slide onto a serving platter.
Cut into wedges and serve!