Place gammon and onions in a large pot. Cover with water, boil, then simmer for 1½ hours.
Preheat oven to 180°C. Remove gammon from the water and cool slightly.
Remove skin and score the fat in diamonds. Place in a roasting tin.
Mix honey, sugar and mustard. Brush over gammon.
Roast for 30 minutes, basting once halfway through.
Rest for 15 minutes before carving. Garnish with rosemary and drizzle with extra honey.