In a small bowl, combine 1/4 cup broth, soy sauce, garlic and minced gingerroot; set aside. In a medium saucepan, combine remaining broth, water and bouillon cube. Bring to a boil over high heat; reduce heat until mixture barely simmers. Crush bouillon cube with back of a spoon. Stir until bouillon cube is completely dissolved. Cover; simmer 5 minutes. Meanshile, peet and thinly slice gingerroot piece; add to hot broth mixture. Cover; simmer over low heat 15 minutes. Add tofu to soy-ginger marinade, turning to coat all sides. Let marinate 15 minutes. Using a slotted spoon, remove and discard sliced gingerroot from bouillon mixture.
Add spinach to simmering broth; add sherry, if desired. Simmer 1 minute longer. Remove tofu from marinade. Place 1 marinaded tofu slice into each of four soup bowls. Ladle 3/4 cup hot broth mixture into each bowl.