Heat the oven to 200C/390F/gas mark 6. Put the chicken in a large bowl with the oil and half a teaspoon of salt, mix together to coat and set aside.
Heat a large frying pan on a medium-high flame, then sear the chicken thighs on one side for three to four minutes, until the skin turns golden brown, then turn over, cook for another three to four minutes and transfer to a clean bowl.
Put the mirin, maple syrup, soy, miso, ginger, garlic, lime zest and juice in a large bowl. Whisk to combine, then add the chicken and stir to coat. Tip the chicken into a high-sided, 25cm x 30cm baking dish, cover tightly with foil and bake for 20 minutes. Remove the foil and, using tongs, raise each chicken leg one by one and put some coriander stems, chillies and spring onion underneath, returning each leg to the pan cooked side down. Bake the chicken uncovered for another 30 minutes, turning the legs again halfway through and basting a couple of times. The chicken will by now be golden brown, sticky and tender, and the chillies and spring onions will be soft.
To serve, put a chicken leg on each plate, surround it with the coriander, chilli and spring onion mix, and drizzle over the sauce.