In large pot, heat oil over medium-high heat. Add onion and celery and sauté until onions are translucent, about 5 minutes. Add garlic and ginger and cook for 2 minutes.
Add tamari or soy sauce to the pot and stir. Add vegetable broth and miso and cook for 5 minutes.
Add rice noodles to the pot and cook for an additional 5 minutes.
Add optional cooked chicken, mushrooms and cook until mushrooms are slightly tender, about 3 - 5 minutes.
Gently stir in bok choy leaves and cook just until wilted.
Spoon into bowls and garnish with sliced green onions and sesame seeds. Serve hot.