Instructions

1. Peel and thinly slice the onions.
2. Slightly brown the butter in a frying pan at low temperature, then add the onion and sugar. Gently cook for about 30 minutes until soft.
3. Pour in the vinegar and wine, then cook on medium heat for 10-15 minutes. When the onions are dark brown and the liquid is reduced to a thick sticky sauce, turn off the heat.
4. Season with salt and pepper. Let it cool and transfer it into a container or immediately spoon it into sterilized jars.
5. Keep the jars in the fridge or in the larder.