Rub off the fuzzy coating on the skins, then core and chop the fruit into chunks. There’s no need to peel unless you prefer a lighter colour — the skins help the butter thicken and deepen in flavour.
Lightly crush the cinnamon, cloves, and allspice using the back of a spoon or mortar and pestle to help release their oils.
Add the chopped quinces, vinegar, water, crushed spices, nutmeg, and salt to an Instant Pot or pressure cooker. Stir to combine.
Lock the lid and cook on High Pressure for 40 minutes. Allow a Natural Release (about 15 minutes) to let the fruit soften fully and the flavours settle.
Carefully fish out the cinnamon stick and any whole spice pieces.
Use a stick blender to purée the quinces directly in the pot until completely smooth.
Stir in extra sugar to taste. Switch the Instant Pot to Sous Vide at 90 degrees, leave the lid off, and leave for 2 hours, stirring every 15–20 minutes, until the mixture is thick, glossy, and holds its shape on a spoon.
Spoon into clean jars. Let cool to room temperature — it will firm up further as it cools. Store in the fridge unless you plan on canning the jars.