Place the flank steak in a shallow baking dish and pour the soy sauce over it. Let marinate, turning occasionally, for at least 15 minutes.
Meanwhile, heat grill to medium-high. In a large bowl, toss the plums, bok choy, oil, ½ teaspoon salt, and ¼ teaspoon pepper.
Grill the steak to desired doneness, 4 to 6 minutes per side for medium-rare. Let rest for 5 minutes before slicing.
Grill the plums and bok choy until tender and charred, about 2 minutes per side. Serve with the steak and sprinkle with the sesame seeds.