Combine beef, onion, seasoning mix and egg in a large bowl.
Add rice and mix well.
Cut peppers in half lengthwise; remove seeds and membrane.
Spread tomatoes on the bottom of an ovenproof 12 X 9 inch baking dish. The liquid from the tomatoes will reduce during cooking so if (like me) you like lots of sauce, add about a cup of water. This also helps make the peppers softer.
Place peppers, cut side up, on top of the tomatoes.
Spoon beef mixture into peppers, mounding as necessary.
Top each pepper with salsa.
Bake in preheated 375°F oven for 40-45 minutes or until beef is cooked thoroughly and peppers are soft.
Cover each pepper with cheese and return to oven, under the broiler, until the cheese melts- less than a minute, so watch carefully!
Serve with sour cream and additional salsa.