Preheat oven to 350°F.
Melt Tuscan Butter in 10½- inch oven-proof skillet over medium heat. Add red pepper and onions. Sauté onions until soft.
Beat eggs, salt and Parmesan cheese together. Pour egg mixture over onions.
Sprinkle mozzarella or cheddar cheese over the top. Place pan in oven.
Bake for 15 minutes.
Slide frittata onto a plate and serve.