Place all the ingredients in a medium pot over medium heat. Stirring occasionally, once the sugar has melted and the mixture begins to bubble, simmer uncovered for an hour or so, stirring occasionally, until the candy thermometer reads 220 degrees F. This doesn't always guaranteed it's ready, so at this point also do a plate test to make sure the mixture is done: Spoon a little of the liquid onto a cold plate and wait a couple of minutes. Push the liquid with your finger and if it wrinkles it is ready. If it doesn't, continue to simmer and re-test. The quince will increasingly change into a deep ruby red color.

Grease a 8x8 inch glass baking dish. Puree the mixture with an immersion blender or blend in a Vitamix or similar until smooth. Pour the hot mixture into the greased baking dish and smooth the top. Let it cool to room temperature, then cover with plastic wrap and refrigerate for 24-48 hours until firm. Invert the membrillo / quince paste onto a platter (you may need to gently pry it out with a knife). Note: Some recipes call for placing the membrillo in an oven at low temp (125 degrees F) for 8+ hours to firm it up. This membrillo is already firm and perfectly sliceable. If you prefer it even firmer, proceed with dehydrating it.Cover with plastic wrap and store in the fridge for up to 3 months or longer. For longer storage membrillo can be frozen well-wrapped for up to a year. Makes one 8x8 inch square of quince paste.Serve with hard and semi-hard cheeses (Manchego is classic) and bread and crackers.