Preheat the oven to 425°F.
Pat the chicken thighs dry with paper towels. Season both sides of each thigh with kosher salt and freshly ground black pepper.
Heat 1 tablespoon olive oil in a large cast-iron (or ovenproof) skillet over medium-high heat until the oil shimmers.
Add the chicken thighs to the skillet skin-side down. Cook until the skin is golden and releases easily, about 4 to 5 minutes. (The oil may pop and splatter — use a splatter screen or an outdoor burner if desired.)
Flip the thighs and cook the meat side for about 4 to 5 minutes. Transfer the chicken to a plate and set aside.
Drain off all but 1 tablespoon of fat from the skillet.
Add the finely chopped onion and minced garlic to the skillet. Stir and scrape the bottom of the pan to loosen any browned bits, and cook, stirring frequently, until the onion is soft, about 3 minutes.
Stir in 2 tablespoons tomato paste and cook for 1 minute, stirring, to deepen the flavor.
Add the drained and rinsed chickpeas, 1/4 cup harissa, and 1/2 cup chicken stock to the skillet. Stir to combine and bring the mixture to a gentle simmer.
Nestle the chicken thighs back into the chickpea mixture with the skin side up, spooning some sauce around them but not over the skin.
Transfer the skillet to the preheated oven and roast until the chicken is cooked through, 20–25 minutes (internal temperature 165°F is recommended). Let the chicken rest briefly before serving.