Trim the base of the bok choy. Then cut the leaves crosswise into 1 1/2-inch-wide strips. Place a pot, large enough to hold all the bok choy, over medium heat. When it is hot, add the olive oil and coat it evenly. When the oil is hot, add the onion and stir-fry until softened, about 2 to 3 minutes. Add the bok choy and season with the salt and pepper. Cover and cook until tender, stirring occasionally, about 10 minutes. Stir in the vinegar and toasted sesame oil, serve hot. Serves 2.