Remove the beef steak from the refrigerator half an hour before cooking to come to room temperature. Thoroughly rinse the two bunches of bok choy under cold running water and remove the tough ends. Cut the greens into large pieces, keeping the firm white stems separate from the tender green leaves.
Heat a skillet over high heat and place the beef steak in it. Sear the meat firmly on both sides and then cook for a few minutes to your desired doneness. Remove the steak from the pan and let it rest covered on a cutting board.
Immediately place a wok over high heat. First stir-fry the white pieces of bok choy for one minute. Then add the green leaves and the tablespoon of sesame seeds. Stir continuously until the leaves have just wilted, while the stems retain their crispy bite.
Slice the rested beef steak with a sharp knife into thin, even slices. Divide the stir-fried bok choy with sesame seeds over a plate and arrange the sliced meat neatly on top to serve right away.