In Sauté Pan or Wok, heat oil over medium-high heat. Add mushrooms; cook until soft, 5 min. Add beef, seasoning, and water. Cook, stirring often and scraping up brown bits from the wok, until meat is cooked through and sauce is gravy-like, about 3–4 min.
Meanwhile, slice peppers in half, lengthwise. Remove stems, pith, and seeds.
Using a spoon, stuff pepper halves with mushroom and beef mixture. Press down and pack peppers. (They will be full!) Top with cheese.
In bottom of Multipurpose Steamer, arrange four pepper halves. Top with tray. Place remaining two peppers halves on top of tray. Cover with lid. (It’s okay if the peppers are loosely covered.) Microwave on high until cheese is melted and peppers are tender-crisp, 5 min.
To serve, carefully remove peppers using tongs.
Per serving: Calories 300, Fat 15 g (Saturated 6 g, Trans 0 g), Cholesterol 60 mg, Sodium 170 mg, Carbohydrate 9 g (Fibre 2 g, Sugars 3 g), Protein 31 g.