Combine ground meat, beaten egg and onion in a large ball and mix well. For children who have egg allergies, use pre-mixed 1/2 tsp starch and 1/2 tsp water to replace the egg. Form into bite-size balls.
In a small bowl, mix sugar, soy sauce, sake, ketchup, vinegar and water to make sauce.
Heat vegetable oil in a large saucepan over medium heat. Stir in meatballs and cook about 5 minutes until outside is brown. Remove excess oil using paper towel, then add the sauce. Cook about 5 minutes more until the liquid is almost gone. Make sure meat is well-done.
Add pre-mixed starch and water to make the leftover sauce thicker.
Meatballs can be kept frozen without changing the texture or flavor. Cook many at once, freeze and defrost a small portion each morning!