Instruction from ianandluis.blogspot.com membrillos
To make membrillos is a two stage process.
Firstly the quince are cooked for 20 minutes whole, in a pressure cooker. When cool enough to handle they are chopped up into smallish chunks ready to be be cooked like you would cook a jam. Sugar is added to the fruit, in a ratio of 1kg of fruit to 750g of sugar.
Some people skin the fruit but we choose not to. We also add a touch of mixed spice which gives the finished membrillo a lightly spiced undertone.
The mixture is cooked until thickened and transparent (less transparent if you did not skin the fruit), 1hour or longer depending on the amount of juice. It is crucial to constantly stir otherwise you will burn the mixture and taint the taste. Glass dishes are ideal for setting the membrillo. Lightly oil the glass dishes and poor the hot mixture into them and allow to completely cool. When cool divide into squares appropriate to your needs.
We wrap ours in cling film then take them to our local Butcher who vacuum packs them for us. As she never accepts payment, we try and give her some home-baked bread or similar in return. The vacuum packed membrillo will last for over a year and can be enjoyed with cheese and wine, a nice home-baked loaf, oat cakes or by itself.