Heat oven to 350* F. Coat a 13x9x2" baking dish with nonstick spray.
In a large nonstick skillet, cook beef, red peppers, onion and garlic over medium heat until meat is cooked through. Drain off excess fat. Stir in 1/2 cup tomatoes, chili powder, cumin, oregano, and cayenne. Gradually stir in water. Bring to a boil; reduce heat and simmer, uncovered, for 5 minutes.
Meanwhile, bring a large kettle of water to a boil. Cut green peppers in half and remove seeds and membranes. Cook peppers in boiling water for 3-5 minutes. Drain; rinse in cold water and drain again. Spoon about 1/3 cup meat mixture into each half.
Place filled pepper halves into coated baking dish. Cover and bake at 350* F for 25-30 minutes or until peppers are crisp-tender and filling is heated through.
Top with remaining chopped tomatoes. Garnish with sour cream, cheese, green onions, and grape tomatoes if desired.