Combine all of the marinade ingredients in a food processor, and process until smooth. Place chicken breasts in a small dish and coat with marinade. Cover and refrigerate for 3 hours.
Preheat grill to medium heat. Brush the onion slices with olive oil. Place chicken and onions on grill, and grill for 10-12 minutes or until done. Transfer cooked chicken and onions to a cutting board. Dice until ½”-1” pieces.
Lay out the 4 tortillas and distribute cheese, chicken, and onions evenly. Fold in half and place on grill over medium heat. Grill for 2-3 minutes per side, or until cheese has fully melted. Cut into wedges and serve with salsa.