1Pat beef dry. Heat oil in a 5-quart heavy pot until hot but not smoking and brown beef on all sides. Add dashi, sake, and white parts of scallions and simmer, covered, skimming froth and turning beef occasionally, until meat is very tender, 1 1/4 to 1 1/2 hours. (Check periodically and add a few tablespoons water if beef becomes less than half submerged.) Add sugar and simmer, covered, 15 minutes longer (more liquid may evaporate at this point, but that's fine).
2Peel potatoes, halving and trimming into ovals if large, and steam, covered, over boiling water until barely tender, about 10 minutes.