Dice the onion and the bacon into very small cubes and put them in a cold pot.
Sauté them lightly for 6-7 minutes, until they start to caramelise.
Add the thyme, apple vinegar, salt and pepper.
Sauté some more util the vinegar cooks completely into the onion and bacon. Remove from heat.
Dice the butter into cubes, add the onions and bacon and mix well and evenly. Season with a little extra salt, pepper and vinegar.
Place the butter in a dish and set it aside to cool.
Serve cold with hot haked celery.