Preheat the oven to 170°C.
Butterfly the chicken: first, remove its backbone using sharp scissors, then put it on a chopping board (breast-side facing upwards) and flatten it by pushing down the breastbone.
Put the chicken onto a rack placed in a baking tray (breast-side facing upwards).
Cut the onion into thin wedges.
Mix the Teriyaki sauce with the sesame oil. Coat the chicken all over with this mixture, then tuck the onion slices underneath it, directly onto the rack.
Pour some water in the tray - you can add some rice wine or mirin, if you like.
Cover the tin with foil, then put it in the oven and roast the chicken for an hour.
Meanwhile cook some rice and let it rest. Mix the glaze and set aside.
After an hour, take the foil off the chicken and coat it with the glaze, using a silicon brush. Scatter the sesame seeds on top. Turn up the oven temperature to 190C and roast the chicken for another 15 minutes or until golden.
Take the tray out of the oven and put it on a large plate. Cover and rest.
Pour the jus into a pan through a sieve, add the spring onion & garlic and any leftover glaze mixture.
Add the beans, bring to the boil and reduce the sauce. Taste, and add soy sauce, Mirin and rice vinegar to create the right balance for your liking. Also, season with salt & pepper, if needed. Cook for 2-3 minutes, then mix in about 2 large cups of cooked rice and the chopped coriander leaves.
Taste and season again, if needed.
Carve the chicken and serve with edamame beans & rice.