Place the frozen meatballs in a 6-quart slow cooker.
Combine the au jus mix, ranch dressing mix, water, and pepperoncini juice in a bowl. Pour the mixture over the meatballs and stir to coat evenly.
Place the butter slices evenly over the top of the meatballs.
Place the pepperoncini peppers evenly on top.
Cover and cook on low for 3-4 hours. Stir every 30 minutes to prevent burning.
Serve hot, spooning the gravy over the meatballs.
Recipe Tips
Stirring every 30 minutes is a small commitment that makes a noticeable difference. It ensures the meatballs on top get as much contact with the sauce as those on the bottom, and it prevents sticking on the sides and bottom of the slow cooker insert.
If you are transporting these to a party or potluck, cook them fully at home and then switch the slow cooker to the WARM setting for the trip. They will hold well for up to two hours on WARM without losing quality.
If the sauce looks thin near the end of cooking, remove the lid for the last 20 to 30 minutes. This allows some of the liquid to evaporate and the sauce to concentrate and take on a slightly glossier finish.