Pre-heat the oven to 220°C.
In a medium bowl, whisk together the melted butter, warm water and miso paste.
Place the onion halves cut-side up in a large roasting dish and position the chicken on top. Truss the legs and pour the miso sauce over the chicken and onions.
Place in the oven and immediately turn the temperature down to 200°C. Roast for 1 hour and 30 minutes, basting the chicken and onions every 20 minutes with the juices from the roasting dish.
When the chicken is cooked through and the juices run clear, remove from the oven. Drizzle with honey and sprinkle over sesame seeds.
Transfer the chicken and onions to a large serving platter. If the miso sauce is still a little thin, reduce over medium heat until it reaches the consistency of gravy. Drizzle the miso sauce over the chicken and onions and serve any extra on the side.
Serve with fresh chilli, spring onions, coriander and a side of seasonal steamed greens.