Prep the slow cooker: Lightly coat the crockpot insert with cooking spray or a thin layer of oil.
This helps with cleanup and prevents sticking.
Season the potatoes: Add halved potatoes and sliced onion to the crockpot. Drizzle with 1 tablespoon of olive oil or butter and sprinkle with 1/2 teaspoon salt and a pinch of pepper. Toss to coat and spread in an even layer.
Make the sauce: In a small bowl, whisk together the chicken broth, remaining olive oil or butter, minced garlic, Dijon mustard, smoked paprika, thyme, rosemary, and 1/2 teaspoon salt.
Stir in the lemon juice.
Season the chicken: Pat chicken dry and sprinkle both sides with the remaining salt and pepper. This step helps the seasoning stick and boosts flavor.
Layer and pour: Place the chicken on top of the potatoes. Pour the sauce evenly over everything, nudging the potatoes so some sauce seeps underneath.
Cook: Cover and cook on Low for 4.5–6 hours or High for 2.5–3.5 hours, until the chicken reaches 165°F and the potatoes are fork-tender.
Thighs are more forgiving if you need the longer end of the range.
Finish and taste: Transfer chicken to a plate to rest for 5 minutes. Stir the potatoes and onions with the juices. Taste and adjust seasoning with extra salt, pepper, or a squeeze of lemon.
Serve: Slice or shred the chicken and spoon over the potatoes.
Ladle some of the cooking liquid on top. Garnish with fresh parsley.