Sauté the sliced onion with a bit of butter, and season with salt and pepper. Cut the cheese. Beat the eggs and season with salt and pepper.
Put the rest of the butter in a square tamagoyaki (Japanese omelet) pan or frying pan, and cook until soft set as shown in the photo. Turn off the heat.
Add the sautéed onion, cheese and ketchup on one side of the omelet as shown in the photo. Turn the heat back on to low-medium.
Fold the omelet in half. Cook for 1 minute, then turn it over and cook another minute (2 minutes total). When it's done, take it out onto a piece of paper towel to cool slightly.
Mix margarine or butter together and spread it on the bread. Cut the omelet in half and place one half on a slice of bread. Cover with the other slice, and place a plate or something on top as a weight to press the bread and filling together.
The sandwich is finished. You can use whatever thickness of bread you like.
Story Behind this Recipe
During Hokkaido's cold winters, onigiri rice balls for breakfast turn hard in the car, so I usually make sandwiches instead. I've tried to make different fillings so that we don't tire of them.