Peel and very thinly slice the red onion(s) and place in a clean and dry glass jar.
Prepare your marinade in a separate bowl by stirring in the olive oil, red wine or balsamic vinegar with the herbs together. Pour the mixture over the onions in the jar and make sure they are covered with the marinade.
Let sit at room temperature to marinate for 24 hours before using. It will keep well in the fridge for a week or so but it keeps for maximum 2-3 days if unrefrigerated.
