Cover and cook the halved plums without stone, onions, garlic and ginger (all diced) with the orange peel in a saucepan with vinegar, sugar, crumbled chilli, salt and mace, covered, for 30 minutes.
Remove the lid and let it boil for 30 minutes while stirring.
Pass through a sieve and let cool down. Then fill into sealable jars. Store in a cool and dark place.