It’s fashionable to refer to this stuff as “onion marmalade” or “onion jam”. “Relish”, “chutney”, or “goop” might be closer. This is great for serving with pâté, cheese or sausages.
In a decent sized frypan, melt 25g butter, and add 1 tbsp mustard seeds. You can also add a pinch of chilli flakes, and/or a whole clove of garlic, peeled and squished but not chopped, which you remove after five mins. Fry gently for about a minute, and then add 500g brown onions, peeled, halved and sliced, well, not finely, but not roughly either. Red onions are good for this as well. Oh, and a pinch of salt.
Fry on a medium heat, moving the onion around until it’s soft and starting to colour. This will take around five minutes. Easier to manipulate the onion with a pair of barbecue tongs.
Once that’s done, add 75mL water and 50g muscovado sugar. This will start boiling almost immediately – reduce the heat so it’s gently burbling to itself, and cover. Leave for 20 mins, stirring occasionally. Be vigilant – if all the water evaporates the sugar will burn.
Now, add 150ml red wine, and 75ml wine or cider vinegar. Bring this back to the boil, then reduce to a simmer. It’ll probably take twenty minutes for the liquid to reduce by half. To test whether it’s done, stick a wooden spoon or spatula into the pan, and drag it along the bottom, to create a trench. Does the liquid immediately rush in to fill the gap? Not done. Does the liquid hesitate slightly, before rushing in? Better. Is the liquid a little reluctant? Done!
Pop this into a clean jar, seal, and leave in the cupboard for about 24 hours before serving. This gives it chance to mellow and mature, as it doesn’t taste very nice the second it has been made. If your jar has been vigorously sterilised, as per jam making, then it will keep for months.