Cook the rice according to the package directions.
Meanwhile, heat the oil in a large skillet over high heat. Season the steak with ¼ teaspoon each salt and pepper.
Cook the steak, turning often, until cooked through, 3 to 4 minutes. Transfer to a plate.
Wipe out the skillet. Add the bok choy and ¼ cup water and simmer, covered, until tender, 2 to 3 minutes.
Meanwhile, in a bowl, mix the soy sauce, vinegar, ginger, and sugar. Add to the bok choy in the skillet and bring to a boil.
Add the steak and any accumulated juices and cook, tossing, until heated through, 1 to 2 minutes. Serve over the rice.