Combine first 5 ingredients in a blender or food processor; process until smooth. Place chicken in a zip-top plastic bag. Add broth mixture. Seal and marinate in refrigerator 2 hours, turning bag occasionally.
Remove chicken from bag; discard marinade. Sprinkle chicken with salt and pepper. Place chicken and onion on grill rack coated with cooking spray. Grill onion 3 minutes on each side or until onions are browned. Grill chicken 6 minutes on each side or until done. Cool 5 minutes. Cut chicken into very thin slices.
Preheat oven to 400°.
Place 1 tortilla on a baking sheet coated with cooking spray. Sprinkle with 3 tablespoons cheese, half of chicken, half of onion, and 1 1/2 teaspoons cilantro. Top with 1 tortilla. Sprinkle with remaining 3 tablespoons cheese, chicken, onion, and 1 1/2 teaspoons cilantro. Top with remaining tortilla. Coat tortilla with cooking spray. Bake at 400° for 15 minutes or until lightly browned. Let stand 5 minutes. Cut into 4 wedges. Top each serving with 1 teaspoon sour cream. Garnish with cilantro sprigs and serve with lime wedges, if desired.