Drain pineapple, reserving juice; set pineapple aside. In a small bowl, combine the pineapple juice, cornstarch, broth, soy sauce, ginger and garlic. Set aside.
Cook your rice.
Meanwhile, in a large skillet, cook the beef over medium heat until no longer pink. Drain and set aside. (original instructions called for cooking veggies first, then beef. I like to do it the other way around.).
Add oil to pan and stir fry the broccoli, mushrooms, red pepper and water chestnuts for 3-5 minutes or until crisp-tender. Remove and set aside.
Put the beef back in the skillet, add the cornstarch mixture and bring to a boil. Cook for 1-2 minutes or until thickened. Stir in the reserved vegetable mixture and pineapple; heat through. Serve over rice.