Place the duck on a baking tray and brush generously with hoisin sauce. Place in an oven preheated to 180°C for 20 minutes, basting with hoisin sauce using a brush during this time until the duck’s skin is crispy. Finely slice the red cabbage and mix with the ranch dressing. Cut the cucumber into very thin sticks and the spring onion into thin slices. Steam the bao buns. Cut the duck into slices approximately 1 cm thick. Fill the bao buns by placing lamb’s lettuce at the base, followed by the duck, red cabbage, cucumber and spring onion. Serve.