In food processor, add cilantro, walnuts, garlic, lemon juice, salt and cheese. Pulse until well chopped.
Add olive oil slowly, pulsing until smooth.
Combine chicken breasts and pesto in large plastic bag or tupperware, making sure chicken breasts get well covered in the pesto.
Let marinate for at least 1 hour.
Heat grill to medium-high heat.
Add chicken breasts and cook until no longer pink through the middle, about 6-8 minutes on each side depending on how thick your chicken is.
The best way to test doneness is a meat thermometer, which chicken should be 165 degrees F.
Enjoy!