Cut the cherry tomatoes in half. Slice the shiitake mushrooms thinly, separating the caps and stems. Slice the green onion diagonally.

Heat sesame oil in a pot over medium heat. Add the shiitake mushrooms and green onion, and sauté until the green onion softens.

Add the seasoning ingredients (sake, mirin, salt, soy sauce, chicken broth powder, and water) and the cherry tomatoes. Bring to a boil, then cover and simmer on low heat for about 2 minutes.

Mix the cornstarch and water to create a slurry. Add it to the pot while stirring, and cook over medium heat until the soup thickens.

Add the vinegar and stir. Pour the beaten egg into the soup in a circular motion. Once the egg floats to the surface, gently stir and serve.