In a large bowl, combine the chicken, harissa (reserve 3tbsp), and crushed red pepper. Cover the bowl and place in the fridge to marinate for 20 minutes.
Preheat your oven to 425 degrees F.
Preheat a large cast iron pan (or two smaller pans -- oven-safe) over medium-high heat. Add the vegetable oil. Once hot, add the chicken, skin side down, working in batches as needed. Cook for about 5 minutes on each side or until blackened and seared. Immediately transfer the pan(s) to the oven and continue to cook the chicken for about 30 minutes or until the chicken is cooked through. Remove from the oven and let stand for a few minutes before serving.
To prepare the spicy sauce, simply combine the reserved harissa and mayo. Serve with the chicken. Garnish with parsley and sea salt flakes and enjoy!