Wash chicken, dry with paper towels then place into an oven bag. Mix together sake, ginger, wasabi, garlic, sesame oil and sea salt then pour over the chicken. Tie the bag then refrigerate overnight.
Pour boiling water over the mushrooms and allow to soak, covered, for 20 minutes. Remove from water, squeeze dry then slice thin. Grill mince until halfway cooked then dice. Saute onion in oil until translucent then mix with mince, mushrooms, rice and pepper in a bowl. Stuff the mix inside the chicken then tie closed with trussing string. Place back in the oven bag then place bag in a baking dish.
In cold oven, add the chicken then turn to 150 degrees C. Roast for 4 hours, shut oven off, allow to rest for 1 hour then remove from oven and serve.