Slice beef very thin, approximately 1/4 to 3/8 inches thick.  Place meat between sheets of plastic wrap and use a meat tenderizer and gently pound meat into uniform pieces.  If you are using skirt steak cut the meat diagonally, it will help make the meat more tender.  Place corn starch in a bowl and dip steak pieces into corn starch and shake off excess corn starch. Allow the corn starch dipped pieces of meat to rest for 5 t0 10 minutes so the coating sticks to the meat.  While the meat is resting you can continue to prep the remaining ingredients.

In a wok, heat vegetable oil and sesame oil.  Saute beef until just done, and the outside begins to crisp.  Remove meat from pan.  You may need to do this a few pieces at a time.  Once all of the meat has been cooked and removed from the pan, add minced ginger and garlic.  Saute ginger and garlic for approximately 60 seconds, the remaining oil should become very fragrant.  Add soy sauce, water, Maggi seasoning, brown sugar, and rice wine vinegar.  Stir until the sauce thickens, return meat to pan, and add quartered button mushrooms to the pan.  Cook for another 60 seconds or so and add half of the chopped scallions.  When serving the Mongolian beef add remaining scallions to the dish.