1. Put the mushrooms and celery in a colander and steam over boiling water for 5 minutes. Turn into a bowl.
2. Put the artichokes in the colander and steam over the boiling water until barely tender, about 12 minutes. Add to the mushroom mixture.
3. Stir in the remaining ingredients, including enough water to cover the vegetables. The relish will keep for a week or more in the refrigerator.