There are many ways to make a frittata. That's the beauty of it. You can add vegetables or cheese or meat or tuna or whatever you like, and it's still a frittata. The only way to screw up a frittata is to turn it upside down on the pan - then it's no longer a frittata, but an omelet.
In a nutshell, a frittata is an Italian egg bake where you break a lot of eggs in a bowl, mix the whites and yolks well, then pour it in a pan, season, and either bake in stovetop or in the oven - or, as I like to do it, first stovetop and then finish in the oven. The technique that I use allows the top to bake as nicely as the bottom without leaving any runny egg mixture.
In my classic frittata I only use butter, eggs, salt, and black pepper. You can add anything you want in the mixture, but make sure that if you use ingredients that require cooking before consuming (frozen vegetables, red meat, chicken etc) that you cook them first on the pan before pouring the egg mixture over it.
My classic, favorite, easy frittata
salt, to taste (optional)
Heat the oven to 200 C / 390 F. Break the eggs to a bowl and whisk to mix and to incorporate some air. Add salt and pepper (and possible other add-ins) according to taste. Melt a good piece of butter on a full-metal skillet on the stove. When the butter has melted, move the skillet to spread the butter all the way up the edges of the skillet. Pour in the egg mixture. Move the skillet to the oven, middle rack. Cook for a few minutes or until the top of the frittata is set. Remove from the oven, cut in half, and serve with extra butter and other garnishes of choice.
A good, filling lunch, ready in minutes! Delicious as hell! Limitless adaptability! A staple in the kitchen!