Place the first five ingredients in a large sauté pan and bring to a boil.
Add the chicken and simmer until the chicken is cooked through.
Add the onion and mushrooms and cook for a few more minutes until the onion is soft.
Pour the egg over the mixture. Do not stir. Cover with a lid and cook for 1 more minute.
Serve over bowls of hot rice.
Top with mitsuba leaves.