Opippi is a tiny udon restaurant a few minutes walk from Kasumigaseki. I ducked in (literally) during lunch time and was greeted by a kind hostess sat me down facing another businessman at a tiny table, probably two feet across. There are about 8 bar seats and 3 tables total.
I tried to scan the menu but realized it was all in kanji and hiragana. Desperate to try some fresh udon, I pointed to one of the few items that didn't have kanji in the name - "kayaku udon." The hostess nodded, indicating that it was a good choice. OK then, we'll see what comes out.
The cold soup (Opippi serves mostly cold udon in the summer) had a light broth, sweet shiitake mushrooms, egg, shredded daikon, nori, and onions. It was delicious. The udon noodles were smooth, thick, and substantial - I could taste the hand made texture in my mouth. I was able to practice my slurping technique, which I have yet to master even though I've lived in this country for almost a year. Very satisfying. The shiitake mushrooms provided a perfect, subtle balance again the light saltiness of the broth.
Unfortunately I'm not sure what else was on the menu, due to my poor Japanese skills. Most of the udon bowls went for about 650 yen, with dishes like tenzaru pushing over 1,000 yen. Very inexpensive.
If you want to take more than one friend to this place, I'd recommend skirting the lunch rush, since things can be quite cramped. Recommended.