Step 1Peel and cut the potatoes into bite-size pieces. Place the potatoes in a bowl of water to keep them from browning. Drain before cooking.
Step 2Peel and slice the onion in half lengthwise, and then cut crosswise into one-half-inch strips.
Step 3Slice the meat into bite-sized pieces.
Step 4Heat the oil in a medium-sized pan over medium heat until hot, then add the onions and saute until they are slightly limp but not browned. Add the meat and saute just until colored. Add the potatoes and cook just until lightly colored, about 2 minutes, stirring frequently.
Step 5Add just enough dashi to submerge the ingredients and increase the heat to high. Stir in the sake, mirin, sugar and 2 tablespoons soy sauce. When the dashi comes to a boil, skim the surface with a slotted spoon to remove the foam, then reduce the heat and gently cook for several minutes until the meat and vegetables are mostly tender. Stir in 2 to 3 tablespoons soy sauce, or to taste, and continue to simmer until the potatoes are cooked through, 4 to 5 minutes.
Step 6Ladle the meat and vegetables into a bowl, pouring the liquid over. Serve immediately.