Cut onions in half, then thinly slice each half and separate the layers into half-moon shapes. Peel and wash potatoes.
Slice them into thin rounds. Rub them on a clean cloth to get them as dry as possible. Heat two tablespoons of olive oil in an eight-inch non-stick frying pan.
When it is smoking hot, add the potatoes. Toss them around in the oil to get a good coating. Lower heat, cover and cook till the potatoes are half done.
Add onions and toss. Add a generous sprinkling of salt and freshly crushed peppercorns. Cook further for two minutes tossing them from time to time so that they do not brown. Set aside to cool.
Break the eggs into a large bowl and, using a fork, whisk them lightly. Add salt and pepper to taste. Add the potato-onion mixture and mix.
Heat the remaining oil in the pan. Pour the egg mixture. Lower heat to minimum immediately. Cover and cook. Draw the edges in gently with a knife to give a firm shape to the omelette.
When the surface looks firm and dry, turn and cook the other side. Cook further for two minutes and remove from heat. Cut into wedges and serve immediately.
Chef TipTo turn over a thick omelette without breaking, place a flat lid or plate over the pan, carefully invert both so that the omelette is on the lid or plate. Put the pan back on the heat and use the palette knife to gently ease the omelette back in.