Prepare the Chicken: Pat the chicken thighs dry with paper towels and season both sides with salt, pepper, and smoked paprika.
Heat the Skillet: In a large cast-iron skillet, heat the olive oil over medium-high heat. Once the oil is hot and shimmering, add the chicken thighs skin-side down. Cook for about 5-7 minutes until the skin is golden brown and crispy. Flip the chicken and cook for an additional 5 minutes.
Make the Sauce: Pour in the apple cider, scraping the bottom of the skillet to deglaze and incorporate all the tasty bits. Stir in the Dijon mustard, apple cider vinegar, and thyme. Bring the mixture to a simmer.
Return the Chicken to the Skillet: Place the chicken thighs back in the skillet, skin-side up, making sure they are mostly submerged in the sauce. Reduce the heat to low, cover, and simmer for about 20-25 minutes until the chicken is cooked through.
Finish with Butter: Once cooked, remove the chicken again and set it aside on a serving plate. Stir the tablespoon of butter into the sauce until melted and glossy.
Serve: Spoon the apple cider sauce and onions over the chicken thighs. Garnish with fresh parsley for color and flavor. Serve hot, paired with roasted vegetables or mashed potatoes for a complete meal.