Everywhere in Japan is known for something. Yesterday, I mentioned Sendai's "love" of beef tongue. Hokkaido has dairy, produce, and seafood. Kushikatsu in Osaka is delicious.
Utsunomiya, a small city north of Tokyo, is known for gyoza. For those who don't know, gyoza is the Japanese name for fried dumplings. They usually contain pork, cabbage, onion, and some other ingredients for flavor.
The place we went to had a plate of 12 gyoza pieces, with 12 different flavors. They were all good, and if I have the opportunity to stop on the way I'll do it again. But I wouldn't recommend going all the way to Utsunomiya just for fried pork dumplings.
Utsunomiya has a few sights around town, and a jazz scene of sorts. It's also a transfer point for JR riders to get to Nikko, so it can serve as a dinner stop on the way back to Tokyo. Note that tourists looking to go to Nikko and not using a JR Rail Pass should probably use the Tobu line from Asakusa. It's faster and cheaper and the Tobu Nikko station is closer to the sights. The bullet train to Utsunomiya takes under an hour from Tokyo Station; using local trains takes two hours. Utsunomiya's modest sights include a shrine, a cathedral, a park, and a reconstructed castle.
If you are traveling between Tokyo and Sendai or further, you should be able to leave the train in Utsunomiya for a while and catch a later train for the rest of your journey. There are some restrictions especially with reserved seats and it depends on the distance you're traveling; it is best to check with an English-speaking attendant to be sure, and for details on how to do this.