Preboil carrots and mushrooms in the boiling water for 3 minutes. Remove from heat and drain, reserving the liquid. Slice the mushrooms.
In a soup pot or wok heat the oil over medium heat. Add the sliced onion and saute for 3 minutes. Add bok choy and cook for 3 minutes more.
Add the carrots, mushrooms, reserved cooking water and broth to the soup pot. Return to a boil and then reduce to medium heta. Stir in tofu cubes or pork, bean sprouts and soy sauce. Add the miso mixture to the soup and stir. Do not boil. Serve over noodles if desired.