Preheat oven to 375°F (190°C). Season chicken thighs generously with salt and pepper.

Heat olive oil in an oven-safe skillet over medium-high heat. Add chicken, skin side down, and sear until golden brown, about 5-7 minutes. Flip and sear the other side for 3-4 minutes. Remove chicken and set aside.

In the same skillet, add sliced onions and sauté until soft and caramelized, about 10 minutes. Add garlic and cook for 1 minute until fragrant.

Stir in apple cider, Dijon mustard, and thyme. Bring to a simmer and cook for 2-3 minutes.

Return chicken thighs to the skillet, skin side up. Spoon some sauce over the chicken.

Transfer skillet to the oven and bake for 25-30 minutes, or until chicken is cooked through and reaches an internal temperature of 165°F (74°C).

Remove chicken and stir butter into the sauce in the skillet to finish. Serve chicken with the apple cider sauce spooned over the top.