Whisk pepperoncini brine, broth, ranch mix, au jus mix, garlic powder, onion powder, and pepper in the slow cooker.
Add meatballs and sliced pepperoncini. Toss to coat evenly in the sauce.
Dot the butter pieces over the top of the meatballs.
Cover and cook on Low 3–4 hours, or High about 2 hours, until hot and tender.
Stir once halfway through cooking to re-coat the meatballs in sauce.
Thicken if desired: mix cornstarch with water, then stir into the sauce. Cook 10 minutes.
Garnish with parsley and serve warm as an appetizer or over rice or mashed potatoes.