Put entire quince and the lemon zest into a pan filled with boiling water and simmer for an hour and a half or until a dark pink colour. Remove the quince from the water, process or blend. Keep the quince water, add the same volume of sugar and turn it into quince syrup for champagne cocktails.
Then push it through a sieve with a wooden spoon. This takes ages but keep going! You could, on the other hand, peel, quarter and core the quinces beforehand. Make sure however that you get out all the really hard grainy bits around the core. In this case tie all the leavings into a cheesecloth/gauze as the pips increase the pectin content.
Weigh the sieved pulp and weigh out exactly the same amount of sugar. Put the pulp, sugar, lemon juice and vanilla into a pan and simmer for an hour, stirring regularly. Take care not to let the volcanic bubbling splatter your hand as you stir. Wear a thick long oven glove.
Pour into dishwasher-clean glass jars or glass jelly moulds.
However you need to ‘dry’ the membrillo. So once the quince membrillo has set, lay out to dry.
If properly dried it should keep for a year in the fridge.