Skin chicken with sharp knife; cut in thin 2 inch long slices. Sprinkle with salt.
Mix cornstarch with 3 tablespoons water and soy sauce. Combine with beef broth and water.
Bring to boil; stir often; add chicken, mushrooms and liquid, vinegar and sugar. Heat. Simmer 5 minutes. Serve with a slice of lemon in each bowl, a cruet of vinegar on the table.