Peel the quinces, remove the core and cut them in 4 slices. Cut the slices in small pieces (3-4 cm thick).

Pour the sugar and the juice of half a lemon over the quinces. Stir and allow to stand for 24 hours, stirring every 3-4 hours.

After 24 hours take only the liquid released by the quinces and pour it into a small pot. Put the pot on at low heat and bring the liquid to the boil, stirring continuously, in order not to let the liquid stick. Once boiled, keep stirring and let it simmer for 4-5 minutes. Put out the fire and pour the hot liquid over the quinces. Leave to stand for 24 hours and repeat the procedure.

After additional 24 hours, take both the juice and the quinces and boil them inside a small pot, stirring continuously. Let simmer for 15-20 minutes, until the liquid gets thick. Put out the fire and leave to cool.

Once cooled, add the mustard oil to taste (about 12-13 drops for each kg of mostarda).

Close the mostarda in airtight jars.