In 12x8-inch (2-quart) glass baking dish, combine 2 tablespoons of the teriyaki sauce, water and soy sauce; mix well. Cut tofu into 8 slices, 1/4 to 1/2 inch thick. Place tofu in dish; turn to coat with sauce mixture. Let stand at room temperature for 15 minutes to marinate, turning occasionally.
Heat oven to 375°F. Cover dish loosely with foil. Bake at 375°F. for 10 to 15 minutes or until thoroughly heated.
Meanwhile, combine remaining 2 tablespoons teriyaki sauce and broth in large nonstick skillet. Add mushrooms and onion; cook over medium heat for 8 to 10 minutes or until vegetables are tender. Serve mushroom mixture over tofu.