Heat 2 tsp olive oil in a large skillet over medium-high heat. Season the tops of the chicken thighs with ½ tsp salt and ¼ tsp pepper. When the oil is hot and shimmering, place the seasoned side down. Cook 4 minutes, then flip. Continue cooking until chicken reaches about 160 °F (about 3–4 more minutes). Remove chicken, cover loosely with foil, and rest.

In a bowl, whisk together the apple cider and Dijon mustard to form your cider mixture. Set aside.

Wipe out (or gently wipe) the skillet, then add the remaining 2 tsp oil. Heat over medium-high. Add the apple slices, remaining ½ tsp salt and ¼ tsp pepper, and the rosemary. Cook ~5 minutes, stirring occasionally, until apples start to soften and take on light golden edges.

Return the chicken to the pan and pour the cider-mustard mixture over. Gently tilt the chicken in the sauce so everything gets coated. Simmer for 2–3 minutes until chicken is cooked through (165 °F) and sauce thickens slightly.

Garnish with extra rosemary and plate the chicken. Spoon the apples and sauce over top to serve.