Trim the root of the shimeji mushrooms and separate them into bite-sized pieces. Slice the onion thinly.

In a plastic bag, combine milk and breadcrumbs, then add ground meat and the remaining meatball seasoning ingredients. Seal the bag and knead until sticky. Cut a small opening at the tip of the bag.

Heat olive oil in a frying pan over medium heat. Add onions and sauté until softened. Push onions to the side, stop the heat, and pipe out 1/10 of the meat mixture into round shapes. Resume heat and cook meatballs, turning until browned, for about 3 minutes.

Add the tomato sauce seasoning and shimeji mushrooms to the pan. Bring to a boil, cover, and simmer on low heat for 15 minutes.