Tofu And Mushroom Onion Skewers With Plum Dipping Sauce

1 cup chicken or beef broth ½ cup sweet rice wine (mirin) ¼ cup reduced-sodium soy sauce 3 tablespoon sugar 1 tablespoon grated fresh ginger 1 tablespoon minced garlic 1 teaspoon fish sauce ½ teaspoon ground black pepper