Preheat the oven to 375°F (190°C).
Season the chicken thighs on both sides with kosher salt and black pepper.
Heat olive oil in a large oven-safe pot 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 another 3 minutes. Remove chicken from the pot and set aside.
Reduce heat to medium. Add diced onion to the pot and sauté for 3-4 minutes until translucent.
Add minced garlic and cook for 1 minute until fragrant.
Stir in the halved baby potatoes and cook for 3 minutes, stirring occasionally.
Sprinkle the flour over the potatoes and onions, stirring well to coat and cook for 1 minute.
Slowly pour in the chicken broth while stirring to avoid lumps.
Add the buttermilk, dried thyme, dried rosemary, and smoked paprika. Stir to combine.
Return the chicken thighs to the pot, nestling them among the potatoes and sauce, skin-side up.
Bring the mixture to a gentle simmer, then cover the pot with a lid or foil.
Transfer the pot to the preheated oven and bake for 30 minutes, until the chicken is cooked through and the potatoes are tender.
Remove the pot from the oven and place it back on the stove over low heat.
Add the unsalted butter and stir gently into the sauce until melted and smooth.
Sprinkle chopped fresh parsley over the casserole before serving.