Aka quince cheese, membrillate (Spanish), cotognato (Italian), pate de coings (French) and marmelata (Portuguese), this has to be one of my favourite things to do with quinces.  It is the classic accompaniment for manchego cheese, but also very good with aged cheddar.

The trick is to avoid graininess, a common flaw.  Quinces have tiny rock-hard grains in their cores, which will pass through just about any sieve.  Most recipes tell you to cook whole quinces then blitz and sieve the lot, but this results in grainy membrillo.  So remove the cores before or after boiling the quinces.  I prefer after, as a) cutting cores out of raw, hard quinces is tricky, and one of these days I will slice right into my hand, and b) the cores and pips help add colour and pectin, so better to leave them in until just before you sieve.

The other tip is to add some acidity in the form of lemon juice or tartaric acid, to balance all that sweetness.