http://www.bestyummyrecipes.com/chimichurri-sauce/

4 boneless, skinless chicken breasts, pounded to 1/2 inch thick
1 1/2 cups fresh parsley
1 cup fresh cilantro
1/3 cup olive oil
1/2 small onion
3 tablespoons red wine vinegar
3 cloves garlic, minced
1 teaspoon salt
1/2 teaspoon black pepper

4 boneless, skinless chicken breasts, pounded to 1/2 inch thick 1 1/2 cups fresh parsley 1 cup fresh cilantro 1/3 cup olive oil 1/2 small onion 3 tablespoons red wine vinegar 3 cloves garlic, minced 1 teaspoon salt 1/2 teaspoon black pepper

Rinse chicken breasts in cold water and pat dry with paper towels. Place on a large cutting board and cover with clear plastic wrap. Take a kitchen mallet and flatten breasts evenly to about 1/2 inch thick.Meanwhile, place remaining ingredients in food processor and blend to make chimichurri sauce. Place chicken breasts into resealable plastic bag and pour chimichurri mixture over top. Make sure to coat chicken well with marinade before sealing bag. Place into refrigerator for 1 to 8 hours in refrigerator. The longer the marinade time, the more flavor will be infused into the flesh.
Preheat grill for medium high heat. Take chicken out of bag and discard marinade. Place chicken onto to grill and cook 5 minutes per side, or until meat is no longer pink and juices run clear. Remove from heat and serve. You can also make ahead of time and serve chilled and sliced over salads or in sandwiches.