Preheat the oven to 425°F (220°C). Pat the chicken thighs dry and season both sides generously 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 shimmering.
Place the chicken thighs skin-side down in the hot skillet and cook until the skin is golden brown and crisp, about 4 to 5 minutes; then flip and sear the other side 1 to 2 minutes. Transfer the chicken to a plate.
Pour off all but 1 tablespoon of fat from the skillet. Add the chopped onion and minced garlic, stirring and scraping the browned bits from the bottom of the pan; cook, stirring frequently, until the onion is softened, about 3 minutes.
Stir in 2 tablespoons tomato paste and cook for 1 minute, then add the drained chickpeas, 1/4 cup harissa, and 1/2 cup chicken stock. Bring the mixture to a gentle simmer.
Nestle the seared chicken thighs skin-side up into the chickpea mixture in the skillet. Transfer the skillet to the preheated oven and roast until the chicken is cooked through and reaches an internal temperature of 165°F, about 20 to 25 minutes.
Remove the skillet from the oven and let the chicken rest a few minutes before serving with the harissa chickpeas spooned over or alongside.