Combine the first 5 ingredients in a bowl and set aside. Place the oil in a large skillet or wok set over medium high heat. Add the beef and cook until nicely browned, then transfer to a bowl.
Add the bell pepper, onion, garlic and ginger to skillet or wok and stir-fry 2-3 minutes. Add the sauce
mixture from the bowl, cashews and chestnuts and bring to a simmer. Return the beef to the skillet or wok, toss to coat with the sauce, heat through 1 to 2 minutes, and then serve.
Note: Sliced stir-fry beef is available in the meat department.