Pat the chicken thighs dry. Season on both sides with 1/2 teaspoon of the salt and the black pepper.

Heat the olive oil in a large 12-inch skillet over medium-high heat. Set the chicken in the pan skin-side down. Sear for 7–8 minutes without moving it much, until the skin is golden. Flip and cook for 2 minutes. Transfer to a plate; the chicken is not fully cooked yet.

Reduce heat to medium. Add the sliced onion and apple wedges to the same skillet. Cook for 4–5 minutes, stirring gently, until the onion softens and the apples begin to color. Stir in the garlic for 30 seconds.

Pour in the apple cider and broth, scraping up the browned bits. Stir in the Dijon and add the thyme. Bring to a gentle simmer. Return the chicken and any juices to the skillet, skin-side up, keeping the skin mostly above the liquid.

Cover loosely and simmer over medium-low heat for 15–20 minutes, or until a thermometer in the thickest part of each thigh, away from bone, reads 165°F (74°C). Remove the lid for the last few minutes if you want the sauce to reduce further. Cooking time varies with thigh size.

Transfer the cooked chicken to a plate if the sauce still needs thickening. Simmer the sauce uncovered for 2–4 minutes, then taste. Add the remaining 1/4 teaspoon salt if needed. If the sauce tastes too sweet, stir in a little of the optional apple cider vinegar, up to 1 teaspoon. Return the chicken to the skillet and spoon sauce around it. Serve with apples, onion, and thyme.