Season the chicken Pat chicken thighs completely dry with paper towels. Mix all dried spices together and rub generously over every surface of the chicken - top, bottom, and under the skin if possible. Season more aggressively than you would for stovetop cooking - slow cooking dilutes seasoning over time.
Optional brown (recommended) Heat a cast iron skillet over medium-high heat until very hot. Add chicken skin side down with no oil - the skin has enough fat. Sear for 2 minutes until golden. Flip and sear 1 minute more. This step is optional but adds a layer of savory flavor that slow cooking alone cannot create.
Build the slow cooker base Place roughly chopped onion and whole garlic cloves on the bottom of the slow cooker. This lifts the chicken slightly above the base and adds flavor to the cooking liquid. Pour chicken stock over the onions.
Add chicken Place browned (or unseared) chicken thighs skin side up on top of the onion and garlic base. Skin side up keeps the skin from sitting in liquid the entire cook, which prevents it from becoming completely soft.
Cook on low Set slow cooker to LOW and cook for 5 to 7 hours. Do not lift the lid during cooking - each peek adds 15 to 20 minutes of cook time. Bone-in thighs are forgiving and can handle the full 7 hours without drying out.
Check temperature At 5 hours, check the internal temperature at the thickest part near the bone without touching it. Thighs are safe at 165F but best at 175 to 185F - at this range the collagen has fully converted to gelatin and the meat pulls cleanly from the bone with almost no effort.
Optional broil for skin If you want texture on the skin, transfer chicken to a sheet pan and broil on high for 3 to 4 minutes. Watch closely - it goes from pale to golden to burnt quickly. This step firms the skin and adds color without any additional cooking of the meat.
Rest and serveRest slow cooker chicken for 5 minutes before serving. Serve over mashed potatoes or jasmine rice with a spoonful of the cooking liquid from the slow cooker poured over the top.