Finely chop the onion and cut the chicken into 1cm cubes.　Heat butter in a frying pan, add onions and fry. Once the onions are cooked, add the chicken.

Stir-fry the chicken and add the leftover rice. If the rice is hard, you can warm it up in the microwave.　Add ketchup, stir-fry, salt and pepper to taste, and set aside.

Add a pinch of salt to the eggs, mix well, and add to a frying pan with butter to make thinly fried eggs.　Cook the eggs slowly over low heat, and once they are cooked, add the ketchup rice on top.
Cover both sides with egg, turn over and place on a plate.　If you find it difficult to flip the omelet, you can make it easier by placing a plate on top of the omelet and then flipping the frying pan.