Preheat the oven to 375°F (190°C).
Season the chicken thighs evenly with kosher salt and black pepper on both sides.
Heat olive oil in a large oven-safe skillet or Dutch oven over medium-high heat.
Add the chicken thighs skin-side down and sear for 5-6 minutes until the skin is golden brown and crispy. Flip and cook for an additional 3 minutes. Remove chicken from the skillet and set aside.
Reduce heat to medium. Add diced onion to the skillet and sauté for 4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Sprinkle the flour over the onions and garlic, stirring constantly for 1 minute to cook the flour.
Slowly whisk in the chicken broth and buttermilk, stirring to combine and create a creamy sauce.
Add dried thyme, dried rosemary, and smoked paprika. Stir well.
Add the halved baby potatoes to the skillet and stir to coat them in the sauce.
Return the chicken thighs to the skillet, placing them skin-side up on top of the potatoes.
Dot the butter evenly over the chicken and potatoes.
Transfer the skillet to the preheated oven and bake uncovered for 35-40 minutes, or until the chicken is cooked through (internal temperature reaches 165°F/74°C) and the potatoes are tender.
Remove from the oven and sprinkle chopped fresh parsley over the casserole before serving.