Mita Seimen is another shop that is capitalizing on the recent boom of tonkotsu gyoukai-based broth places, most famously Rokurinsha and Tetsu. This place just recently opened and everyone working there was very new and seemingly inexperienced, but still hustling quickly. The chashu was cold (but not rolled, instead the cut type sliced from a larger slab, decent fat-to-meat ratio) but actually pretty good after you warmed it up in the broth, which was almost an exact clone of Tetsu's broth in my opinion. A good balance of flavor and texture, not too watery or salty. This is good because this place does not have much of a line (yet) - if you are craving the Tetsu or Rokurinsha broth taste but don't want to wait, then come here. The noodles were very mochi-mochi, well-cooked and a generous portion, you can see the menu here. They even have a white lantern hanging out front just like Rokurinsha. There's a second floor and a dumbwaiter but it wasn't clear if the second floor was open yet when I went. After you finish the counter staff will bring your bill over to the register for you (there is no ticket machine). Even if you are familar with the area around JR Tamachi Station or Tokyo Metro Mita Station, I still recommend that you print out and bring the map as the shop is in an area that has a lot of narrow little streets and is a bit confusing.
Amasan review page
Diddlefinger Map (English labels)