In a large skillet, cook the beef, red pepper, onion over medium heat until meat is no longer pink. Add garlic; cook 1 minute longer. Drain.
Stir in 1/2 cup tomatoes, chili powder, cornstarch, cumin, oregano and cayenne. Gradually stir in 1/2 cup water. Bring to a boil. Reduce heat; simmer, uncovered, for 15-20 minutes.
Meanwhile, cut green peppers in half lengthwise; remove seeds. Place in an ungreased shallow 3 qt. microwave-safe dish; add remaining water. Cover and microwave on high for 8-10 minutes or until crisp tender. Drain; fill each pepper half with 1/3 cup beef mixture.
Top with the remaining tomatoes. Garnish with sour cream, cheese, green onions and grape tomatoes if desired.