Prep the chicken. If the thighs have skin, remove it so the sauce doesn’t get greasy.
Pat the chicken dry and sprinkle lightly with black pepper.
Layer the slow cooker. Add the sliced onion to the bottom of the slow cooker, if using. Place the chicken thighs in a single layer on top. Scatter the minced garlic over the chicken.
Add seasonings. Sprinkle the ranch seasoning and au jus mix evenly over the thighs.
Nestle the pepperoncini around the chicken and pour in the pepperoncini brine.
Finish with liquid and butter. Pour in the chicken broth. Dot the top with the butter pieces.
Cook low and slow. Cover and cook on Low for 5–6 hours or on High for 2.5–3.5 hours, until the chicken is tender and easily pulls away from the bone.
Shred or serve whole. For sandwiches or bowls, pull the chicken off the bone and shred it into the sauce. For a plated meal, leave thighs whole and spoon sauce over the top.
Adjust and garnish. Taste the sauce.
If it’s too salty, add a splash of water or a squeeze of lemon. If you want more tang, add a bit more pepperoncini brine. Garnish with chopped parsley.
Serve. Pair with mashed potatoes, rice, buttered noodles, polenta, or toasted rolls.
A simple green salad or roasted vegetables rounds it out.