For the tea-smoked duck, preheat the oven to 180C/350F/Gas 4.
Line the base of a wok with a double layer of aluminium foil and place the tea leaves and soft brown sugar on top. Place a trivet on top, cover with a lid and heat until smoking. Place the duck breast on the trivet, cover the wok and smoke the duck for 12 minutes. Remove from the wok, place on a baking sheet and roast in the oven until just still pink in the middle. (NB: tea-smoking generates a great deal of smoke. Be certain to work in a well-ventilated area.)
For the mash, place the chopped sweet potato into a pan of boiling salted water and cook until tender. Drain well and then pass through a ricer or mash with a potato masher. Add the soy sauce, lime juice, salt and freshly ground black pepper and mix well.
For the onion rings, pour the vegetable oil into a large saucepan and heat until a small cube of bread turns golden in 30 seconds. (CAUTION: Do not leave hot oil unattended.) Dip the onion rings into the beaten egg and then into the flour and then carefully lower into the hot oil and fry until golden-brown. Remove with a slotted spoon and drain on kitchen paper. Season to taste with salt and freshly ground black pepper.
To serve, pile the mash onto a serving plate, thinly slice the duck and place on top, and arrange the onion rings on top of the duck.