Place bao buns in a single layer over paper. Steam according to packet directions, until soft and spongy. Combine char siu sauce with 1 tablespoon water in a heatproof jug. Microwave on High (100%) for about 30 seconds, or until warm. Serve buns filled with pork, cucumber and onions. Drizzle with sauce. Garnish with fresh coriander sprigs.