In a small bowl, prepare the teriyaki sauce by putting together the soy sauce, honey, and mirin. Mix well until the honey has completely melted. Set aside. In a second bowl, another small one, soak the bread in milk. When the bread has absorbed the milk, lightly squeeze out the milk and drain.
In a third bowl, a larger one, mix the beef, egg, salt, 1/4 teaspoon pepper, 1 teaspoon sesame seeds, sesame oil, onions, and the soaked bread. Mix with the hands until the mixture becomes compact.
Make little balls with the meat. In a non-stick skillet, over medium heat, in a saucepan with the olive oil (leave a tablespoon for later use), brown the meatballs. Over medium heat, in another saucepan with the remaining olive oil, transfer the meatballs. Add the teriyaki sauce and cook for about 3 minutes. Season with the remaining pepper.
Garnish with the green onions and the remaining sesame seeds.