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 5-7 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, add the halved potatoes and sliced onion. Cook for 5 minutes, stirring occasionally, until the onions soften and the potatoes begin to brown.
Add the minced garlic and cook for 1 minute until fragrant.
Sprinkle the flour over the potatoes and onions, stirring to coat evenly. Cook for 1-2 minutes to eliminate the raw flour taste.
Slowly pour in the chicken broth and buttermilk while stirring constantly to create a creamy sauce.
Add the dried thyme, dried rosemary, and smoked paprika. Stir to combine.
Return the chicken thighs to the skillet, placing them on top of the potatoes and onions, skin-side up.
Dot the butter evenly over the top of the casserole.
Transfer the skillet to the preheated oven and bake uncovered for 30-35 minutes, or until the chicken is cooked through (internal temperature reaches 165°F/74°C) and the potatoes are tender.
Remove the skillet from the oven and sprinkle the grated Parmesan cheese over the casserole.
Return to the oven for 3-5 minutes until the cheese is melted and slightly golden.
Garnish with chopped fresh parsley before serving.