Preheat the oven to 200°C.
Season the pork fillet with salt and pepper.
Sear the pork in a hot ovenproof pan with oil until golden-brown on all sides.
Roast the pork in the oven for 6 to 8 minutes and let it rest for 5 minutes.
Dissolve sugar with 40g butter, lemon juice, and 500ml water in a saucepan.
Simmer the quince in the liquid until softened then strain, reserving some liquid.
Blend half the quince into a smooth purée and pass through a sieve.
Season the purée in a clean pan and cover until needed.
Fry the remaining quince segments in the remaining butter for 2 minutes.
Wilt the shredded chard in a pan with butter.
Slice the pork and serve with purée, fried quince, chard, and jus.