In a medium bowl add the garlic, rosemary, thyme, salt, pepper, chili flakes, olive oil and lemon juice and mix well.
Generously season the chicken with salt and pepper and pour the marinade mixture to coat completely, massage for 1 minute.
Cover the bottom of the cast iron skillet with some of the sliced onions, and add the chicken skin side down.
Put the rest of the onions on top and cover the pan with plastic wrap, transfer to the fridge and let it marinate for 24 hours.
Take the chicken, onions and marinade out of the skillet and transfer to a plate.
Heat the skillet on the stove over medium-high heat with 1 tablespoon of olive oil, when hot, place the chicken skin side down and sear for about 5 minutes or until the skin turns golden brown. Turn the heat off and remove the chicken.
Add the onions back to the bottom of the skillet and put the chicken skin side up on top of the onions and add the marinade and pancetta slices on top.
Transfer to the oven and roast at 400° for 35 to 40 minutes or until internal temperature reaches 165°.