For the Dressing: Wisk ponzu, shallot, whole grain mustard, olive oil, and wasabi (remember its optional) in a bowl thoroughly. Slice steak as thin as possible with a sharp knife. Remove excess fat as needed. Peel and thinly slice cucumber (remove seeds if necessary). Slice scallions thin for garnish. To Serve: Arrange steak slices evenly over a serving platter. Lay cucumber slices on top. Spoon dressing over the dish (you may not need all of it) and sprinkle with scallions. Season with a little salt. Serve immediately. Leftover dressing can be stored in the refrigerator for up to 1 week.