Put the broth in a saucepan; add the mushrooms and spinach and simmer 3-4 minutes. Combine the soy sauce, vinegar, pepper, sesame oil and pepper oil in a small bowl; stir to blend, then add to the broth. Taste and adjust seasonings. Add the tofu and the cornstarch-water mixture, stirring constantly. Continue to cook until soup thickens. Sprinkle cilantro and scallion on top of soup for garnish. Makes 6 cups.