Marinade the minced beef in soy sauce, caster sugar and cornflour for 5 minutes.
Add the beaten egg and stir in one direction until the mixture becomes sticky. Whilst stirring, add enough water to make the beef mixture smooth. Take small portions of the beef mixture and form into meatballs.
Heat a large frying pan over medium heat and cook the meatballs until done, about 5-7 minutes (depending on the size of the meatballs). Drain and set aside.
Heat the same frying pan with 30g butter over medium heat. Saute the onion, carrots and potatoes until fragrant, about 3-4 minutes, then transfer into a large saucepan.
Add just enough water to cover the ingredients and bring to the boil. Turn down to medium heat, cover and cook until the potatoes are soft, about 15 minutes.
Turn off the heat and remove the saucepan from the hob. Add the curry concentrate and meatballs. Stir until the sauce has thickened. Cover and allow to rest for 10 minutes before serving.
Japanese soy sauce (tamari) and Japanese curry concentrate can be purchased in Oriental speciality stores or online.