Prepare shells and cheese according to package directions in a medium saucepan.
Process diced tomatoes and green chiles in a food processor or blender until minced. Stir minced tomatoes into prepared shells and cheese in pan. Stir together sour cream and chopped cilantro; add to shells and cheese mixture, stirring until well blended.
Coat a 2-qt. baking dish with cooking spray. Pour shells and cheese mixture into dish; sprinkle with shredded Cheddar cheese.
Microwave mixture at HIGH 2 to 4 minutes or until cheese is melted. Top with tomato slices and cilantro sprigs, if desired.