As with omelets, you can put anything you want in a frittata. But you can make a frittata big enough to share and do the whole thing in one cast iron pan. A typical frittata that I make is a frittata Loraine. It’s similar to a quiche Loraine but without the crust.
I start by sauteeing onions and pieces of precooked bacon in a cast iron pan. When that is ready, just scramble some eggs and poor the mixture into the pan with the bacon and onions. Then put pieces of Swiss cheese on top and transfer it to the oven to bake for about 15 minutes into a fluffy frittata. The size pan and number of eggs depends on how many you are making it for. For two people, I usually use 6 eggs and the smaller cast iron pan (8”?) so the frittata will be thicker and not too flat. I’d use the 12” pan and more eggs for 3 or 4 people.
I’ve never done this in any other kind of pan but cast iron. You can cut clean pie-shaped pieces out of it and it never sticks to the pan. If you made more than you need, it reheats nicely for another meal.
I love frittatas. I bet your BFF makes frittatas.