Heat the oil in a large, heavy-based saucepan and sauté the onions slowly over a very low heat, stirring occasionally until they are soft (about 25 minutes).
Add the remaining ingredients and simmer gently until the juices thicken. Add salt and freshly ground pepper to taste and set aside for eight hours to develop the flavours.
Pour into dry, clean and warm jars and store in the refrigerator.
Serve with roast beef, ham or cheeses.
Makes about 4 jars.
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