Peel and thickly slice potatoes and boil until just tender. Slice onion into rings. Heat olive oil in a large, deep frying pan. Sauté the onions over a low to medium heat for about 15 minutes. Add potatoes and toss to coat. Cover and sauté for a further 10?15 minutes. Season well.
Beat eggs with a fork; pour over potato mixture and agitate to combine. Cook over a medium heat (but don't stir) until the base is deep golden and firm and the top is almost set ? for about 15?20 minutes. Loosen omelettes and place a plate over the pan. Invert the pan and, with a bit of luck, the omelettes will plop right onto the plate! Then slide them back into the pan to cook the other side. Drizzle a little oil down the sides of the omelettes and cook over high heat until golden and firm, about 5 minutes. Remove, slide onto a plate and cool. Slice into wedges and eat while still warm.
Note: Cooking the onions slowly is very important ? this brings out their sweetness, which makes the dish particularly divine. Slice, pile and tuck right in. Or, if the whole thing is too much for you to co-ordinate, simply cook on the stove-top (to set the bottom) and then grill the top (to set the rest of the egg).