Preheat the oven to 375°F (190°C).
Season the chicken thighs evenly with kosher salt and black pepper on both sides.
Heat the 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 crisp. Flip and cook for another 3 minutes. Remove chicken from the skillet and set aside.
Add the halved baby Yukon gold potatoes and sliced onion to the skillet. Cook for 5 minutes, stirring occasionally, until the potatoes start to brown and the onions soften.
Add the minced garlic and cook for 1 minute until fragrant.
In a small bowl, whisk together the buttermilk, chicken broth, dried thyme, smoked paprika, and all-purpose flour until smooth.
Pour the buttermilk mixture over the potatoes and onions in the skillet. Stir gently to combine and bring to a simmer.
Return the chicken thighs to the skillet, placing them skin-side up on top of the potatoes.
Dot the unsalted butter evenly over the top of the chicken and potatoes.
Sprinkle the grated Parmesan cheese over the entire casserole.
Transfer the skillet to the preheated oven and bake uncovered for 30 minutes, or until the chicken is cooked through and the potatoes are tender.
Remove the skillet from the oven and let the casserole rest for 5 minutes.
Garnish with chopped fresh parsley before serving.