Preheat broiler. In a bowl, whisk first 4 ingredients. Add bok choy; toss to coat. Arrange bok choy, cut side up, on a rimmed baking sheet (reserve remaining dressing). Spray bok choy with cooking spray. Broil until charred in spots, 8 to 10 minutes. Arrange on a plate, drizzle with remaining dressing and sprinkle with sesame seeds.