Put the snow peas in a small saucepan with enough water to cover; bring to a boil and immediately remove from heat. Drain and set aside.
Heat the oil in a large skillet over medium heat; cook the beef in the oil until browned. Add the potatoes; cook and stir until soft, 5 to 7 minutes. Stir the dashi soup, soya sauce, sake, and sugar into the mixture; simmer for 10 minutes.
Reduce heat to low and scatter the chopped onion over the mixture; allow to simmer until the liquid is nearly completely evaporated, about 15 minutes more. Top the mixture with the snow peas to serve.