Stone and halve the plums. Peel and finely dice the onion and ginger. Rub the peel of the orange (alternatively, 1 tablespoon of ground, dried orange peel can be used).
Bring all ingredients to the boil together and reduce to a thick consistency over a low heat. In between, briefly puree the chutney (so that the plums become smaller).
It can take up to 3 hours to boil down, stir regularly at the end - it can burn.
Pour hot into twist-off glasses and let cool upside down.