Combine first 4 ingredients in a small saucepan; bring to a boil. Cover, reduce heat, and simmer 40 minutes or until done. Stir in chopped parsley, black pepper, salt, and tomatoes. Cook 3 minutes. Remove from heat.
Preheat oven to 400°.
Spoon about 3 tablespoons rice mixture into each piquillo pepper. Place stuffed peppers in an 8-inch square baking dish coated with cooking spray. Sprinkle evenly with Manchego cheese. Bake at 400° for 10 minutes or until peppers are thoroughly heated. Garnish with parsley sprigs, if desired.