Cut bok choy in half lengthwise, leaving core intact. Steam bok choy, covered, 4 minutes or until tender; drain well.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Drizzle cut sides of bok choy with oil; sprinkle with salt and pepper. Place bok choy, cut sides down, in pan; cook 6 minutes or until lightly browned. Turn bok choy over; cook an additional 1 to 2 minutes or until lightly browned. Sprinkle with sesame seeds.