Prep the slow cooker: Lightly coat the insert with nonstick spray or a thin layer of oil. This helps prevent sticking and makes cleanup easier.
Season the chicken: Pat the chicken dry.
In a small bowl, mix salt, pepper, onion powder, garlic powder, paprika, parsley, and dill. Rub the seasoning all over the chicken.
Build flavor in the cooker: Add the chopped onion and garlic to the bottom of the slow cooker. Place the seasoned chicken on top.
Tuck in the butter.
Add liquid: Pour in the chicken broth. Cover and cook on Low for 4 to 6 hours or High for 2.5 to 3.5 hours, until the chicken is tender and easily shreds.
Make the sour cream mixture: In a bowl, whisk sour cream with cornstarch and lemon juice (or Dijon). This keeps the sauce from curdling and adds a silky texture.
Thicken and finish: When the chicken is done, transfer it to a plate and tent with foil.
Turn the slow cooker to High. Whisk the sour cream mixture into the hot cooking liquid. Cook uncovered for about 10–15 minutes, stirring occasionally, until slightly thickened.
Shred and return: Shred the chicken with two forks.
Return it to the slow cooker and stir to coat in the creamy sauce. Taste and adjust seasoning with salt and pepper.
Serve: Garnish with fresh parsley or chives. Serve over rice, egg noodles, mashed potatoes, or steamed vegetables.