To make the sauce, mix the hot water and sugar in a large roasting dish, stirring to allow the sugar to melt and the water to cool. This should only take a few minutes. Add the soy sauce, ginger, garlic and chilli flakes, if using, and mix well.
Add the chicken, cover and leave to marinate in the fridge for 30 minutes. The chicken doesn’t have to marinate at all if you are in a rush but, if there is time, marinate in the sauce overnight so the sauce really soaks into the chicken. Preheat the oven to 200C/180C Fan/Gas 6.
Roast the chicken for 25 minutes, or until cooked through and the juices run clear with no trace of pink when the thickest part is pierced with a skewer. Take the chicken thighs out of the roasting dish, leaving the marinade behind.
Add the 5 nests of noodles to the roasting dish, flipping them over to coat them in the liquid. If the marinade has become quite dry, add up to another 300ml/10fl oz hot water.
Put the chicken back on top of each noodle nest and put back into the oven for 10 minutes to allow the noodles to absorb the moisture. Garnish with the coriander, spring onions, red chilli and sesame seeds and serve immediately.