How to Make It
Combine 1/4 cup chopped cilantro and next 4 ingredients (through garlic) in a large bowl. Reserve 1/4 cup marinade in a medium bowl. Add 1/4 teaspoon salt and chicken to remaining marinade, turning to coat. Let stand at room temperature for 20 minutes, turning occasionally.
Heat a grill pan over high heat. Coat pan with cooking spray. Add onion slices to pan; grill 10 minutes, turning occasionally. Add 1/4 teaspoon salt and onions to reserved 1/4 cup marinade; cover tightly with plastic wrap. Let stand 10 minutes.
Return grill pan to high heat. Coat pan with cooking spray. Remove chicken from marinade; discard marinade. Sprinkle chicken evenly with remaining 1/4 teaspoon salt. Add chicken to pan; grill 5 minutes on each side or until done. Serve chicken with onion mixture, avocado slices, and lime wedges. Garnish with chopped cilantro, if desired.