Combine the soy, sake, mirin and sugar in a small pot and reduce by half then cool.
Place the chicken into a large baking dish and using poultry scissors cut down either side of the backbone and remove, butterflying the chicken. Then pour over half the sauce (the rest is for basting while cooking later) along with the salt and 1 tbsp of sesame oil and rub into the chicken all over. Allow to marinate for at least 30 minutes at room temperature before cooking, or overnight in the fridge.
Preheat an oven to 190C.
Tuck the onions underneath the chicken, in the bottom of a baking dish, mixing with some marinade then add ½ cup of water to the dish. Don’t pour this over the chicken.
Roast for 40 minutes, using a pastry brush to baste the chicken with the reserved marinade throughout the cooking every 10 minutes. Once the chicken is ready, allow it to rest for 5 minutes before cutting into smaller pieces.
While the chicken is resting, make the cucumber salad. Use the side of a large chef’s knife and crush the cucumbers until the burst, then roughly chop them and place into a bowl. Add the salt and toss around then add the garlic, coriander, vinegar and sesame oil and toss to combine.
Cut the chicken into joints (smaller pieces) using poultry scissors or a large knife with all the onions and cooking juices poured over. Serve with the cucumber salad and steamed rice if desired.