Preheat the oven to 160°C. Place the sweet potato in a large saucepan, cover with plenty of water and bring to the boil. Simmer for 12-15 minutes, until tender. Drain, then mash with three-quarters of the oil, plus the maple syrup and soy sauce.
Meanwhile, place the remaining oil in a heavy ovenproof frying pan over a high heat. Season the chicken and fry for three minutes on each side until golden. Put the pan in the oven for 20-25 minutes until thoroughly cooked, the juices run clear and there is no pink meat. Remove, cover with parchment paper and rest for four to five minutes.
Mix the lemon zest, garlic and parsley together. Spoon the mash onto plates, top with the chicken then scatter over the parsley mixture. Drizzle over a little more olive oil, if using.