Hold the bouquet of parsley by the stems and remove (shave off) parsley leaves. Then coarsely chop. In a food processor, add parsley and garlic, process until finely chopped. Add olive oil, red wine vinegar, oregano, red pepper flakes and salt. Give a few pulses just to combine until chunky sauce forms.
In a large bowl, add chicken and most of the chimichurri sauce, reserving some for serving. Cover and refrigerate for 30 minutes to 24 hours. Discard marinade before cooking. When ready to cook, you can grill, bake or pan fry the chicken.
Grill: Preheat grill on medium-high (450-500 degrees F) and rub with oiled paper towel. Place chicken on the grill, close the lid and grill chicken thighs for 10-12 minutes or chicken breasts for 8-10 minutes turning once. Do not overcook. Remove chicken from the grill, cover with foil and let rest for 5 minutes.
Bake: In a large baking dish, add marinated chicken in a single layer. Bake uncovered in 450 degrees F preheated oven for 25-35 minutes or until internal read thermometer registers 150 degrees F internal temperature. You do not need to wait for 165 degrees F as meat will get there during resting. This way you will avoid dry chicken.
Pan Fry: Cook chicken in preheated skillet on medium heat for 5-6 minutes per side or until chicken is cooked through. Keep in mind that pieces of parsley will burn a bit and that's fine.
Serve chimichurri chicken warm topped with remaining chimichurri sauce.
Notes
Make ahead: Allow chicken to marinate with the chimichurri sauce overnight, covered in the refrigerator, or freeze it with the marinade (without marinating in the fridge first).
Store: Refrigerate chimichurri chicken in an airtight container for up to 5 days.
Freeze: Transfer cooked and cooled chicken to freezer-safe container and freeze for up to 2 months. Thaw in the refrigerator overnight before enjoying cold or reheating.