Bring a large pot of lightly salted water to a boil. Add the mussels and simmer until the shells just open, 1–2 minutes. Remove from the heat and drain. Discard any shells that do not open. Remove and discard one of the shells from each mussel.
In a wok, bring the Soy Dressing ingredients to a boil over medium heat. Add the beef and simmer uncovered for 2–3 min