Remove and discard the stems of the shiitakes. Wipe the shiitakes clean with a damp paper towel. Cut the shiitake caps into thin slices. In a medium saucepot, heat the canola oil. Add the shiitakes and saute until soft. Pour in the broth and bring to a boil.. Add the wontons and cook about 3 minutes. Reduce the heat and stir in the soy sauce and sesame oil, if using; Napa cabbage and green onions. Simmer 2 minutes. Remove from heat and ladle into bowls. Serves 4 generously.