Heat oil in a nonstick skillet on medium heat.
Add sliced Onion and bell pepper and saute for 3 or 4 minutes.
If using Mushrooms, basil and/or garlic, add them now. (Spray Mushrooms briefly with cooking spray.) Saute another minute or two. Don't allow the garlic to brown.
Add salt and pepper to eggs and stir. Don't completely combine.
Pour into the skillet over the veggies and continue to cook until set on the bottom.
To help the frittata release, you may wish to spray the sides of the pan above the omelet with more cooking spray.
Lift the edges as they set, allowing raw egg to run underneath. When it won't run any more, be sure it is loose and will slide.
If the handle of your pan isn't oven-proof, you must slide the omelet out onto an oven proof platter. Otherwise leave it in the pan.
Arrange the tomatoes on top the omelet. Sprinkle with the Cheese.
Place as far as possible from the flame of your broiler until the Cheese melts. Offer extra black pepper and serve with whole Wheat toast, or if the family is off and running, wrap a chunk up in a tortilla or stuff it in a pita.
More at http://sneakykitchen.com/Recipes/ Cheese_frittata.htm