Negi Scallion: Since I did not have a Japanese "Negi" scallion, I used a wedge of onion. I cooked it slowly in a frying pan with a bit of light olive oil, turning over once or twice for 10 minutes or until nice char marks developed on both sides and the onion is cooked.
Broth: I used one "dashi pack (The one I used had kelp and dried bonito, but no dried fish)" in water (about 1.5 cups) and simmered for 5 minutes to make dashi. Any dashi, including instant granulated ones, will do. I added mirin (1tbs) and soy sauce (2 tbs to taste, I could have added more in retrospect). I kept it just barely simmering or hot.
Duck breast: The leftover duck breast had nicely browned skin and was cooked to medium rare. I cut thinly (1/4 inch) and then dusted the pieces with potato starch, katakuriko 片栗粉. I placed each piece in the simmering broth (above) for 20-30 seconds so that the starch cooks into a slightly slippery coating on the surface of the meat. It also very slightly thickens the broth. if I was cooking the duck from scratch, I would cook the skin side only in a frying pan rendering as much fat as possible while making the skin brown following the first step of my usual way of cooking the duck breast. Instead of finishing the duck in the oven, I would slice the meat and cook it in the broth as descried above for a slightly longer time. You can omit the potato starch, if you do not like the slippery texture.
Soba noodle: I just used dried soba and cooked as per the package instruction, washed in running water, drained and placed in the center of an individual serving bowl.
Assembly: In the bowl with soba noodle on the bottom, I added the broth, and arranged the onion and duck meat as seen above. I garnished it with chopped green onion. Just before eating, I sprinkled 7 flavored Japanese red pepper powder 七味唐辛子.
Up until this point, we were enjoying Orin-Swift "The Prisoner" 2009 (Zin and Cab mix). It was certainly a good wine but it is not as good as the prior vintages and we prefer "Papillon" 2007 (Bordeaux blend with predominant Cab) from the same winery. But this dish cries out for sake and we obliged. I should have added a bit more soy sauce to the broth but otherwise it was a very nice dish and indeed went very well with cold sake.
We also enjoyed stewed "Kabocha" squash. As before this one was sold as "Butter cup" squash but I believe this is identical to a Japanese "Kabocha".