Peperoncini Chicken

Name: Peperoncini Chicken
Ingredients: 2 lb. skin-on, bone-in chicken thighs (6–8); 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, divided; ¾ tsp. freshly ground pepper, divided; 3 Tbsp. extra-virgin olive oil, divided; 1 lb. fingerling potatoes, halved lengthwise; 10 peperoncini, plus ½ cup brine; 2 garlic cloves, finely grated; 1 tsp. dried oregano; ½ medium red onion, thinly sliced; 3 celery stalks, thinly sliced (about 1 cup), plus celery leaves for serving; 2 Tbsp. chopped parsley