Heat the oil in a large frying pan and then gently fry the sliced potatoes until almost soft, stirring from time to time so that they don't burn on the bottom of the pan. Add the onion and the green pepper and continue frying until all the vegetables are soft. Drain the vegetables in a colander to get rid of the excess oil.
After about ten minutes, turn the omelette by placing either a flat plate or saucepan lid on the frying pan and quickly turning over. Gently slide the omelette back into the frying pan and continue frying, once again shaking the pan from time to time so that it doesn't stick to the bottom, until it has set all the way through.