Combine the hoisin sauce, water, bean sauce, vinegar, onions, and sugar in a pan over moderate heat and simmer until the onions are soft, about 15 minutes. Add the peanut butter and stir well. The sauce should be just thick enough to coat a spoon. If it is too thick, add a little water. Set aside to cool.
When serving, transfer to an individual sauce bowl and garnish with chili paste and chopped peanuts. This sauce will keep up to two weeks if refrigerated.