My mother-in-law handed me this recipe on a napkin at my bridal shower.

Preheat your oven to 400°F (200°C). Place a large cast iron skillet on the middle rack while the oven heats so it gets nice and hot.
Pat the chicken thighs very dry with paper towels. This helps the skin get crispy instead of steaming.
In a large bowl, stir together the dry onion soup mix and olive oil until it forms a loose, grainy paste.
Add the chicken thighs to the bowl and toss well, using your hands to rub the onion mixture all over the chicken, getting it under the skin where you can. Make sure every piece is evenly coated.
Carefully remove the hot cast iron skillet from the oven and place it on the stove or a heat-safe surface. Arrange the chicken thighs in the skillet skin-side up, leaving a little space between each piece so the skin can crisp.
Place the skillet back in the oven and bake for 35 to 45 minutes, or until the chicken skin is deep golden brown and crispy and the internal temperature reaches 175°F to 185°F when checked in the thickest part (avoiding the bone).
If you want extra crispy skin, turn the oven to broil for the last 2 to 3 minutes, watching closely so the onion coating doesn’t burn.
Remove the skillet from the oven and let the chicken rest for 5 to 10 minutes. Spoon some of the caramelized onion drippings from the pan over the chicken just before serving.
Variations & Tips