Tsukune (Japanese Chicken Meatballs)

Finely chop the onions.
Place the ground chicken and salt in a bowl and knead the mixture thoroughly for a few minutes, until it becomes sticky and holds together. Then, add the chopped onions and egg, and mix well until combined.
Lightly coat a pan with oil, using a paper towel to spread it evenly over the entire surface.
Take a portion of the chicken mixture and shape it into a ball or an oval patty about 1 inch (2.5 cm) thick. Place the shaped pieces in the pan as you make them, making sure they don't touch.
Carefully flip them to help them keep their shape, and cook the other side until browned as well. Then cover the pan, reduce the heat to medium-low, and cook for about 4 minutes.
Remove the lid and turn off the heat. Add the sauce ingredients (sake, mirin, sugar, and soy sauce), then return the pan to medium heat. Simmer until the sauce has slightly thickened, occasionally spooning it over the tsukune (meatballs).
Transfer the tsukune to a serving plate. They are delicious on their own, but you can also enjoy them wrapped in shiso leaves or dipped in raw egg yolk, if desired.