Step 1Wipe mushrooms clean with damp paper towel. Break stem off each and set aside. Thinly slice caps, then stack them into a pile and cut in half. Rinse spinach in cold water and dry well. Trim roots and most of green tops from onions and thinly slice. Set aside.
Step 2Mix soy sauce, vinegar, pepper, sesame oil and hot pepper sauce. Set aside.
Step 3Combine broth, mushrooms and spinach in large pot. Simmer over medium heat 4 minutes.
Step 4Stir soy sauce mixture into broth. Taste and add more soy sauce or hot sauce--whatever you feel would improve and balance flavor.
Step 5Mix cornstarch and water and add to broth. Stir a couple of minutes to blend. Add tofu and continue to stir; cornstarch mixture will give a slight thickness to broth. Serve hot (or cold) with a sprinkle of chopped green onions on top.