Trim beef of visible fat. Slice in 1/4-inch slices. I used the thick, boneless flat chuck roast (it looks like a very thick chuck steak).
Line the crock of a 5 qt slow cooker with a cooking bag or spray with non-stick spray.
Heat 1 tbs of the oil in a large heavy skillet over high heat. When oil just begins to smoke, at the beef slices, making sure not to overcrowd the pan. Brown well on both sides. Repeat if you had meat that you couldn't fit the first time. Place browned beef in the slow cooker.
Heat the other 1 tbs oil over medium-high heat. Add the mushrooms, water chestnuts and onion. Saute until onion begins to get soft. Add the soy sauce, ginger and the sherry (or mirin), scraping up all the browned bits at the bottom of the pan. Place the mushroom mixture on top of the meat in the slow cooker.
Drain the mandarin orange slices. Set the drained sections aside. Mix reserved mandarin orange syrup with the cornstarch, mixing well. Add the beef stock to the orange/cornstarch mixture; stir well. Pour over meat and vegetables in the slow cooker.
Cover and cook on HI 4-5 hours or on LOW 8-10 hours. Mix in the reserved mandarin orange sections immediately before serving. Top with chopped green onion.
Serve over rice (I used jasmine rice).