Prep the aromatics. Slice the onion, mince the garlic, and slice the apples into wedges. If using carrots and celery, cut them now.
Layer the base. Add onion, garlic, apples, and any veggies to the bottom of the crockpot. Sprinkle with a pinch of salt and pepper.
Season the chicken. Pat chicken dry and season both sides with salt, pepper, and half the cinnamon.
If using dried thyme, sprinkle it over the chicken now.
Whisk the sauce. In a bowl, mix apple cider, chicken broth, Dijon, apple cider vinegar, remaining cinnamon, and nutmeg. Taste; if very tart, stir in a small drizzle of maple syrup.
Combine. Place chicken on top of the aromatics. Tuck in fresh thyme and rosemary.
Pour the cider mixture over everything.
Cook low and slow. Cover and cook on Low for 4 to 6 hours (thighs) or 3 to 4 hours (breasts), until the chicken is tender and reaches 165°F internally. Avoid lifting the lid frequently.
Optional browning. For extra flavor, you can sear the seasoned chicken in 1 tablespoon oil or butter in a hot skillet for 2 to 3 minutes per side before adding to the crockpot.
Thicken the sauce (optional). Remove chicken to a plate and tent with foil. Stir 1 tablespoon cornstarch with 1 tablespoon cold water.
Turn the crockpot to High, whisk in the slurry, and cook 10 to 15 minutes until slightly thickened. Return chicken to warm through.
Finish and serve. Taste and adjust seasoning with salt, pepper, or an extra splash of vinegar for brightness. Serve over mashed potatoes, rice, cauliflower mash, polenta, or crusty bread to soak up the sauce.