Peel the potatoes, remove the sprouts and cook them in a pot of boiling water with salt. Once cooked, mash them while they are still hot with a potato masher or a fork. Chop the onion and your beef into small pieces. The more chunks you have, the more chewy your Korokke will be. If you prefer a fine texture, use a food processor at this stage.
Heat oil in a skillet over medium heat and brown the ground beef. When it starts to change color, add the chopped onion and sauté again. While stirring, add salt, pepper and a little nutmeg. When the meat is cooked and the onions are tender, remove from the heat and let cool.
In a large bowl, mix the mashed potatoes and the previous preparation (ground meat and onions). Divide into 4 equal parts and shape into oval balls with your hands.
In a large bowl, mix the beaten egg, water and flour. Spread your panko on another large plate. Take your korokke, dip them first in the egg mixture and then coat them with panko.
Pour about 3 cm of oil from the bottom of the pan, heat to 200°C and fry your Korokke for about 3 minutes until they are golden brown. Once out of the oil, drain them gently in absorbent paper. Arrange them on a plate with tonkatsu sauce, some tomatoes and grated cabbage! Enjoy them hot!