Finely dice the onions. Finely chop the celery stalk and cranberries. Grind the mustard seeds into a fine powder.
Heat the oil in a sufficiently large saucepan and sweat the onion cubes in it. Add celery and vegetables, squeeze the garlic cloves and sauté briefly.
Stir in the remaining ingredients and let the sauce simmer for 1/2 to 3/4 hour until thick enough to the desired consistency. Stir frequently, especially towards the end of the boiling time, and reduce the heat a little if necessary.
As soon as the sauce has reached the desired consistency, season again to taste. Pour hot into a screw-top jar and close tightly. Immediately place the jar on the lid for 10 minutes, let it cool completely and let it steep in the refrigerator for at least 6 hours, but preferably 1 day.
The sauce is easy to prepare and will last at least 1 week in the refrigerator.
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