Wash the quinces and wipe off the soft down. Slice roughly into a casserole. Cover tightly and bake in a preheated oven, 140°C/275°F/gas mark 1, for 1–2 hours, until very tender. Pass the quinces through the fine blade of a mouli légumes, or sieve. 450 g/1 lb quinces yields 300–450 ml/½–¾ pint purée.