Lightly oil a large non-stick pan. Add the potatoes and cook for 5 minutes. Add the onions and continue cooking for another 4-5 minutes. Add the mixture to the eggs, season and mix well together.
Reheat the pan again with a little oil. Add the egg mixture. Cook until the under side is set. Put a large plate on top of the pan and turn the omelette onto it. Then slide it back into the pan and cook the other side. (If the pan is not super non-stick just finish under the grill).
To make the Salsa: Mix all the ingredients well together and set aside until ready to serve.