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 the chicken from the skillet and set aside.
In the same skillet, reduce heat to medium and add the sliced onion. Sauté for 4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the halved baby potatoes to the skillet and stir to combine with the onions and garlic.
Sprinkle the flour over the potatoes and stir well to coat evenly. Cook for 1-2 minutes to remove the raw flour taste.
Slowly pour in the chicken broth while stirring constantly to avoid lumps.
Add the buttermilk, dried thyme, dried rosemary, and smoked paprika. Stir to combine and bring the mixture to a gentle simmer.
Return the chicken thighs to the skillet, placing them skin-side up on top of the potatoes.
Dot the butter evenly over the top of the chicken and potatoes.
Transfer the skillet to the preheated oven and bake uncovered for 30 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.