A frittata is rather easy to make, with one slight tricky part.
You start by sauteing your ‘stuffing’ ingredients in a skillet. (Although Angelina never used one, a non-stick skillet is perfect for making frittate.) For this frittata, slice onions thinly from head to base and saute them very gently in olive oil.
Then add thinly sliced waxy potatoes, turn a few times to let them insaporire, then cover and allow to simmer very gently until they are tender. (If the potatoes are sliced thinly enough, this should not take more than 5 minutes or so.)
Meanwhile, in a large bowl, beat eggs (I usually use six) with grated pecorino cheese (or parmesan, if you like a milder flavor), salt, pepper and, if you like, some chopped parsley. Now you have two options: you can add the cooled potatoes and onions to the bowl, mix gently (a rubber spatula works nicely for this operation) and then slip the contents of the bowl back into the skillet, in which you will have added a bit more olive oil. Or–and this is what Angelina used to do–you can simply pour the egg mixture over the potatoes and onions in the skillet, then lift the potatoes slightly to allow the eggs to run under them and cook.
In either case, let the eggs cook over medium-low heat until they have solidied and formed a nice brown ‘crust’ underneath. (You can use a spatula again to take a ‘peek’ underneath.)
Now comes the tricky part: you need to cook the still-raw top of the frittata. There are different ways to do it. The most common—and, to my mind, still the best way—is to slide the frittata on to a plate, then cover it with another plate, flip it over and slide the frittata, uncooked side down, back into the skillet, having added un filo d’olio to the skillet beforehand. Another way to do it is to cover the skillet directly with the plate and then flip the frittata over. (I find it a bit too risky handling the hot skillet like this, but you can find special frittata pans-which are actually two interlocking non-stick skillet-specially designed for this operation.) And finally, the easier way is to run the frittata under a broiler so that the top cooks and then browns. I use this last method sometimes myself, but it never quite comes out as nice and crispy and golden brown as when you fry it—although you can hide that fact by flipping the frittata before you serve it.
You can serve your frittata immediately, but I find that it has nicer flavor at room temperature or at least having slightly cooled off, say for 5 minutes before eating. Cut it into wedges like a pie to serve.