Heat oven to 375°F. Cook rice in water as directed on package.
Meanwhile, in 12-inch nonstick skillet, brown ground beef over medium-high heat until thoroughly cooked, stirring frequently. Drain. Add corn, chiles and egg; mix well.
Stir in cooked rice and 1 cup of the cheese. Cut bell peppers in half lengthwise; remove seeds and membrane. Place cut side up in ungreased 13x9-inch (3-quart) glass baking dish. Spoon about 3/4 cup beef mixture into each pepper half, mounding as necessary. Top each with 2 tablespoons salsa. Cover tightly with foil.
Bake at 375°F. for 35 to 40 minutes or until peppers are crisp-tender. Uncover; sprinkle with remaining 1/2 cup cheese and the onions. Bake uncovered an additional 5 to 10 minutes or until cheese is melted.