Cut steak into bite-size pieces. Add to the slow cooker, steak, and the remaining ingredients, except cornstarch, water, and mandarin oranges. Cover and cook on low 6 to 8 hours.
About 15 minutes before the end of cooking time combine cornstarch with 1/4 cup liquid from mandarin oranges, stir until smooth and add to the slow cooker along with the oranges. Discard the remaining liquid from the oranges, unless you prefer a thinner sauce, in that case, add the remaining liquid. Stir and continue cooking 15 minutes.
Mandarin Beef is delicious served over brown rice or quinoa.