• Place oven-safe skillet in the oven and preheat it to 450°. I used a cast iron skillet.
• Combine salt and pepper in a bowl.
• Pat chicken with paper towel until completely dry. Rub oil all over it until it's completely covered. Sprinkle the salt mixture all over and rub in with your hands. Gently separate the skin from the breast and rub the seasonings under there as well.
• Tie the legs together with the twine and tuck the wings under the back.
• Place in the preheated pan and cook until the breast is 120°- about 20-25 minutes.
• Turn the oven off and keep cooking until the breast is 160° and the legs are 175°- about 25-35 minutes.
• Remove from pan and let rest 20 minutes.

For the pan sauce:

• While the chicken rests, remove all but 1 tablespoon on fat from the pan (I didn't remove any because there wasn't much).
• Place skillet over medium-high heat and add onion. Cook until it is softened- about 3-4 minutes.
• Add broth and mustard and scrape the bottom of the pan to loosen any brown bits. Simmer until sauce is reduced to ¾ cup- about 3-4 minutes.
• Remove from heat and stir in tarragon, lemon juice and ghee. Serve over chicken.