5poundwhole chicken(giblets removed and discarded, or saved for another time)
Mince the onion and garlic in a food processor. Add the parsley, oregano, red pepper flakes, salt and pepper, and process everything until the parsley is finely chopped and all ingredients are incorporated. (Scrape the bowl of the food processor a few times if necessary.
Scrape the mixture into a medium bowl and add to it the olive oil, vinegar, lemon juice and lemon zest. Stir everything together - this is your chimichurri sauce! This will give you about 1-1/4 cups of lemon chimichurri.
Remove 1/2 cup of the chimichurri and set it aside. This will be to serve with the finished chicken.
Place the chicken in a shallow dish or pan. Pat the chicken dry with paper towels. Tuck the wings back behind the body. Place the sliced lemons into the cavity of the chicken, then tie the legs together with kitchen twine.
Use the remaining chimichurri sauce on the chicken: pour some sauce over the bird and rub it into the entire surface. (I like to use my fingers to do this but you can also use a basting brush.) Pour on a little more as you need to so that the entire bird is coated. (If you have leftover chimichurri you can serve it later as long as you keep it free of utensils and fingers that have touched raw chicken.) Loosely drape a piece of plastic wrap over the chicken, put it in the fridge and allow it to marinate for 30 minutes.
Preheat the oven to 450 degrees F. Place a cast iron skillet large enough to hold the chicken into the oven, so that it heats up with the oven.
Remove the chicken from the fridge. Carefully lift the chicken from the dish and place it into the hot cast iron skillet sitting on the oven rack. Pour any marinade from the pan over the bird, and close the oven. Roast the chicken for 15 minutes per pound - 1 hour 15 minutes for a 5-pound bird. *Tent the top of the chicken with a loose piece of foil for the last 30 minutes, to keep the chimichurri from burning too much. Use a meat thermometer to check if it's done: the thickest part of the breast should be 175 degrees F and dark meat should be 180 degrees F. (Be sure you're not touching bone with the thermometer.) If the chicken isn't done roast it for additional 10 minute intervals until it's ready.
Remove the pan with the chicken from the oven and allow it to rest for 10 minutes before carving. Serve the chicken with the reserved chimichurri for dipping or spooning over pieces.
If time is crunched you can skip the marinating step and just put the chicken in the oven right away. If you don't have a cast iron skillet use a baking dish or roasting pan instead.