Slice the onion thinly. Chop the mitsuba into 3 cm pieces. Beat the egg in a bowl.

Cut the grilled eel into bite-sized pieces.

In a pot, combine mirin, soy sauce, and water. Bring to a boil, then add the onion and simmer over medium heat until softened.

Add the grilled eel to the pot, spreading it evenly. Simmer for 1-2 minutes, then pour the beaten egg over the mixture. Cook until the egg is semi-cooked, then turn off the heat and add the mitsuba.