Wash and stone the plums. Peel and halve onions and cut them into strips. Heat oil in a coated pan. Fry the onions gently over medium heat for about 2 minutes.
Mix sugar, nectar and fruit vinegar. Add plums to the onions and fry for 3-4 minutes. In the meantime, peel and finely chop the ginger. Deglaze plums with nectar mixture. Add mustard seeds, cloves and juniper and boil down everything, stirring occasionally, until a viscous mixture is formed.
Fill hot into prepared, sterilized jars and close immediately. Relish tastes great with cheese and baguette, fondues, grilled food and various rice dishes.