Wash, peel and core quinces and cut into slices or chunks. For every 1 kilo of peeled fruit, add only 200ml of water. It’s not much water, which is why I do the following stewing step in the oven with the lid on instead of on top of a stove. I’d only burn it if I cooked it on top.
I put my fruit and water in a large stainless steel boiler, with the lid on, in a moderate oven (160 to 170 deg.) for however long it takes for the fruit to go soft and pulpy. If I’m not going to look at it often, I use a slower oven. If I’m in the kitchen and watching it, I use a higher temperature. Takes maybe an hour, perhaps longer. Just make sure all the pieces are pulpy. Cool a little, then put the whole lot through a food processor.
Put the pulp back in the boiler with 450 grams of sugar and the juice of 1 lemon for each original 1 kilo of quince fruit. Stir until sugar is well incorporated.
Put in a slow oven – about 120 degrees I find OK – without the lid on it, for a long while – maybe 14 hours. Stir every couple of hours, but I have been known to leave it overnight at 110 deg without attending to it, just giving it a good stir to dissolve the top drier bit back into the bulk of the pulp when I get up. Stir it every couple of hours.
Leave it in the oven cooking until it turns a really dark colour, and is starting to come away from the sides. Don’t think “that will do”, but wait until you can see that it is coming away from the sides of the pan. If you like, you can test a bit in a cold saucer put in the freezer for a few minutes.
Line a large, flat baking tray (I use a large biscuit tray I have for my large oven) with a sheet or two of Glad Bake. Pour the cooked pulp into the tray and smooth over the top with a spatula. Leave somewhere to continue to dry out a bit for a couple of days. It should just come away from the bottom paper lining in a soft, rubbery slab. Cut into small squares and store in sealed containers. Freezes well