Quinces are ideal fruits for jellies and jams because they have a wonderfully perfumed flesh and, most importantly, because they are full of pectin. In Greece we also make a sweet preserve with pieces of quince, but we also use them in cooking, for example in festive stews with pork or game, the same way you would use potatoes.
In different areas of the country quince jelly is flavored with rose geranium ( Pelargonium graveolens) or lemon verbena, but you can also flavor it with cinnamon and cloves, vanilla, or something to your own taste.
This jelly, popular all over the planet, can be used as a jam on toasted bread, to accompany cheese on a cheese plate, or as a natural jelly to gloss fruit tarts. It has a wonderful red color and, properly preserved, can be stored for up to a year.
Just a general rule:
For every100ml of juice use 85gr granulated sugar
First wash as many quinces as you wish to use without bothering to weigh them because you are only interested in the amount of juice you'll get after cooking them.
Chop the whole quinces (without peeling or seeding them) in pieces and put them in a pot big enough to hold the amount of fruit you use, and pour enough water just to cover them.
Bring to a boil and cook until very soft, about 30-40min.
Pour the cooked pulp into a fine sieve (or cheesecloth) over a pot and let it strain for 4-5 hours or, even better, overnight.
Measure the amount of juice and pour it in a cooking pot together with some lemon juice (1 lemon for every litre of juice), the sugar needed (see above), the flavorings -if used- and bring it to a boil over medium high heat.
The jelly is ready when it reaches the setting point. If you happen to have a cooking thermometer, then the gelling point is at 104 C ( 219 F). If not, find it by pouring a tea spoon of jelly onto a cooled plate and when (after 30 seconds) wrinkles form on the surface when touching it with your finger, it is set. Pour the jelly into sterilized jars and store for up to a year.