Season the chicken thighs with the salt, pepper, and garlic powder.
Heat the oil in a large skillet over medium-high heat.
Sear the chicken thighs skin-side down for 3–4 minutes until golden brown. Flip and cook for another 2 minutes.
Place the chicken thighs in the bottom of a 6-quart slow cooker.
Top with the apple slices.
In a bowl, whisk together the apple cider, brown sugar, Dijon mustard, apple cider vinegar, onion powder, and cinnamon.
Pour the apple cider mixture around the chicken.
Cover and cook on LOW for 5–6 hours or HIGH for 3–4 hours, until the chicken is tender and cooked through.
Carefully transfer the chicken to a plate and tent with foil.
In a small bowl whisk together the cornstarch and cold water until smooth.
Stir the slurry into the slow cooker.
Turn the slow cooker to HIGH and cook uncovered for 15–20 minutes until the sauce thickens into a glossy glaze.
Return the chicken to the slow cooker and spoon the glaze over the chicken.
Sprinkle with parsley before serving.
Serve with mashed potatoes, rice, or roasted vegetables.