Cut the plums into fine cubes. Bring to the boil briefly together with the whiskey, balsamic vinegar, bay leaf and sugar.
Grind the mustard seeds, allspice and juniper, then mix with the charcoal powder and add to the plum-whiskey-balsamic vinegar mixture and stir well. Add olive oil and season with salt, pepper and nutmeg as required. Take out the bay leaf.
Let the mustard steep in the refrigerator for 1 day.