Season chicken thighs with salt and pepper.
Heat olive oil in a large skillet over medium-high heat. Sear chicken thighs, skin-side down, until golden brown, about 4 to 5 minutes per side.
In a bowl, whisk together apple cider, Dijon mustard, honey, minced garlic, and fresh thyme.
Place the seared chicken thighs in the slow cooker.
Add sliced apples to the slow cooker.
Pour the apple cider mixture over the chicken and apples.
Cover and cook on LOW for 4 to 5 hours or on HIGH for 2 to 3 hours, until the chicken is tender and cooked through.
Spoon the glaze over the chicken before serving.