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.
Didn't have time for the marinade, just cooked the chicken and onions in paprika and garlic and added chopped jalapenos to each layer. Watch the oven temp, 160 fan worked better than my first effort! Sour cream and hummus made the dish. Very dramatic starter at a dinner party!
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