Cut Steak into stir fry slices and coat with corn starch. Combine 1/4 cup soy sauce and 1/4 cup sherry in a large mixing bowl (Reserving remaining soy sauce and sherry for last step). Refrigerate for at least 5 mins. Using a wok heat peanut oil on high heat. Stir Fry Steak (Save juices remaining in bowl) until meat is pink (about 2 mins. Put meat in a dry bowl (put aside). Save 2 tbsp of the oil (put aside). Stir Fry Mushrooms with 1 tbsp of fresh peanut oil and fresh soy sauce to taste (drain and put aside). Add the 2 tbsp of oil that was put aside and stir fry ginger for 30 secs to a 1 min. Add watercress and stir fry for 30 sec. Add Meat, Mushrooms and rest of sauce (plus remaining 1/4 cup soy sauce and 1/4 cup sherry). Stir Fry together for 1 min. Serve over rice.