Step 1

Preheat oven to 425°. Sprinkle chicken thighs all over with 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt and ½ tsp. pepper. Coat a large cast-iron skillet with 1 Tbsp. oil. Arrange chicken thighs, skin side down, in pan and place over medium heat. Cook, undisturbed, until skin underneath is browned, 14–16 minutes.

Step 2

Meanwhile, toss potatoes, peperoncini, garlic, oregano, remaining 2 Tbsp. oil, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and ¼ tsp. pepper in a large bowl. Set aside.

Step 3

Turn chicken over. Add potato mixture to pan, arranging snugly around thighs. Pour peperoncini brine around edges of pan and give pan a shake to evenly distribute. Transfer skillet to oven and roast until chicken is cooked through and potatoes are tender, 40–45 minutes.

Step 4

Remove skillet from oven and transfer chicken and peperoncini to a plate. Place skillet over high heat and cook potatoes, stirring often, until coated in drippings and glossy, about 4 minutes. Remove from heat and stir in red onion, sliced celery, and parsley.

Step 5

Place chicken and peperoncini on top of vegetables and scatter some celery leaves over.