Preheat the oven to 400°F (200°C).
Pat the chicken thighs dry and season both sides evenly with the kosher salt and black pepper; set aside.
Heat 2 tablespoons of olive oil in a 12-inch oven-safe skillet over medium-high heat until shimmering.
Add the chicken thighs skin-side down and sear until the skin is golden and crispy, about 8–10 minutes; transfer the chicken to a plate.
If the skillet is dry, add 1 additional tablespoon of oil, then add the diced onion and sauté over medium heat until soft and translucent, about 4–5 minutes.
Add the minced garlic, minced rosemary, and cubed soppressata and sauté briefly until the garlic is fragrant and the sausage is warmed, about 1–2 minutes.
Pour in the chicken stock and stir to deglaze the pan, scraping up any browned bits.
Stir in the drained cannellini beans, then nestle the seared chicken thighs on top of the beans in the skillet.
Transfer the skillet to the preheated oven and bake until the chicken is cooked through and reaches an internal temperature of 165°F (74°C), about 30 minutes.
Remove from the oven and let rest a few minutes before serving.