Heat olive oil in a skillet and sear the chicken breasts on both sides until golden, about 2–3 minutes per side.
Place the sliced onion and garlic in the bottom of the crockpot.
Arrange the seared chicken on top of the onion and garlic.
Whisk together apple cider, Dijon mustard, apple cider vinegar, thyme, salt, and pepper. Pour over the chicken.
Cover and cook on low for 4–5 hours or on high for 2–3 hours, until the chicken is tender.
If you’d like a thicker sauce, whisk cornstarch with water, stir it into the crockpot juices, and cook for another 10 minutes.
Serve the chicken with your favorite sides, spooning plenty of sauce over the top.