From the orange, grate 1 teaspoon of peel and squeeze 1/3 cup of juice.
In a large bowl, whisk together orange peel and juice, sherry (or juice), soy sauce and cornstarch; add beef slices and toss to coat; set aside.
Heat half of the oil in a large skillet over medium-high heat; add bell peppers and 2 tablespoons of water; cook, stirring frequently, for 5 minutes or until peppers are tender-crisp; transfer to a medium bowl.
In the same skillet, heat remaining oil over high heat. With a slotted spoon, transfer half of the beef to the skillet, reserving orange peel/juice mixture.
Cook and stir beef for 1 minute then transfer to the bowl of bell peppers.
Repeat with remaining beef. Add remaining water and reserved orange peel/juice mixture to the skillet; boil for 1 minute.
Toss beef and peppers with orange sauce before serving.