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TO MAKE THE JELLY:
Put the quinces and lemon zest and juice in a large heavy-based pan. Add enough water to cover the fruit. Bring to the boil, then reduce the heat and simmer for about 1 hr until the quinces are very tender. Strain through a jelly bag or muslin-lined colander. Do not press out the juice – just leave it for at least 4 hrs, or ideally overnight, until the juice has dripped through.
Put 3 saucers in the freezer. Measure the juice and return it to the pan (reserve the leftover fruit for the paste). For each 1 litre juice, add 750g sugar (or for each 100ml juice, add 75g sugar). Bring to the boil, stirring to dissolve the sugar, then boil hard until set. This could take between 5 and 20 mins. To test for set, spoon a little onto a chilled saucer, leave for 1 min, then push your finger through the juice. If it wrinkles, the jelly is set. If not, return to the heat and boil again for a few mins, then test again.
Remove from the heat and skim off any surface scum with a slotted spoon. Stir in the butter to dissolve any leftover sediment, then stir in the rose water. Pot into sterilised jars, seal and label. Store in a cool, dry place for up to 1 year.
TO MAKE THE MEMBRILLO PASTE:
Line a 16 x 24cm shallow oblong tin or tray with baking parchment. Tip the reserved fruit into a food processor and whizz to make a smooth-ish paste. Weigh the paste and for each 1kg paste, add 750g sugar (or for each 100g paste, add 75g sugar).
Tip the paste and sugar into a large heavy-based pan and heat gently, stirring to dissolve the sugar. Increase the heat and cook, stirring regularly, for 30-45 mins until the paste is thick and golden, and leaves a gap when you run a wooden spoon across the base of the pan. Be careful as the mixture will spit at you and can burn. Spoon into the tin or tray and smooth the top. Leave to cool, then chill overnight until it is very firm.
Wrap the paste in fresh baking parchment, then over-wrap in foil and store in the fridge for up to 6 weeks.