Pat the chicken thighs dry and season them all over with the kosher salt. Let them sit at room temperature for 10 minutes while you prep everything else.

Heat the olive oil in a large ovenproof skillet over medium-high. Lay the thighs in skin side down and sear them undisturbed for 8 minutes, until the skin is deep gold and lifts off the pan on its own. Turn them and cook 2 minutes more, then move them to a plate.

Pour off all but a tablespoon of the fat. Lower the heat to medium, add the onion, and cook until soft and translucent, about 5 minutes. Add the garlic and cook 1 minute more, just until fragrant.

Stir in the rice and toast it in the fat for 2 minutes. Pour in the chicken broth and the lemon juice, then scrape up every brown bit stuck to the bottom of the pan.

Nestle the chicken back in skin side up, bring it to a simmer, then cover and cook over low heat for 20 minutes, until the rice is tender and the liquid is gone. Keep the lid on the whole time (see notes).

Off the heat, dot the cold butter over the rice and let everything rest, covered, for 5 minutes. Finish with the lemon zest and the parsley, and serve straight from the pan.