Heat olive oil in a large skillet over medium heat.
Season chicken thighs with salt and pepper, then add to the skillet, skin-side down.
Sear chicken for about 5-7 minutes, then flip and cook for another 5 minutes until browned.
Remove chicken and set aside. Add chopped onion to the skillet.
Cook the onions until translucent, then add the sliced apples and cook for an additional 5 minutes.
Stir in the apple cider and bring to a simmer, scraping the bottom of the pan for flavor.
Return the chicken to the skillet, reduce heat and cover, cooking for 25 minutes.
Add Dijon mustard and heavy cream, stir gently to combine.
Cook uncovered for an additional 10 minutes to thicken the sauce.
Taste and adjust seasoning with salt and pepper before serving.