Well, not sure when an omelette ends and a tortilla begins!
slice potatoes thinly - you may want to par boil them so that they are tender slice any other veg gently fry them all in an omelette or smallish frying pan (about 6-8 inches across) season with salt and pepper, herbs if you like/have them
when they are softened, pour 4-6 beaten eggs (number really depends on how many people/how big your pan is) and cook slowly over a low heat
To finish, put the pan under the grill (this makes sure that the egg is cooked all the way through). You could put the feta on the top at this stage, so it melts/cooks.
When it is cooked, turn out on to a plate and slice. Lovely served warm or cold.