This soup was wonderful, although I should note that I made a lot of substitutions. I didn't have bok choy or bean sprouts, so in addition to the fennel I added a lot of additional carrots. As the recipe notes, this would be good with pretty much any veggies you have around. The key to the flavors are the dried mushrooms, the ginger, the soy sauce, and the scallions---so I think I would consider them NECESSARY for the soup, but everything else could be substituted. Oh yeah, I also didn't have a cinnamon stick, so I used one shake of powdered (worked fine--smelled very interesting there at the beginning!), and I didn't star anise, so we went without.
I decided to add Asian noodles (I used Japanese somen noodles) so I could tell the kids it was "Asian chicken noodle soup." The 9-year-old liked it; the 5-year-old found it tolerable after adding Parmesan cheese (in her mind, if there are noodles, there should be parm....ah well, she's only 5).
I had never served fennel to my family before, and worried that it would taste too strongly of licorice (which my family likes, but I didn't know how they'd feel about it for dinner)--but no one said anything about it; I guess the cooking mellows the flavor.
The sesame oil and sriracha sauce were wonderful additions. Anyway, I loved it. Thanks EW!