Adjust oven rack to upper-middle position and preheat oven to 475°F (246°C). Line 2 large rimmed baking sheets with aluminum foil and spray with nonstick cooking spray.
In a large mixing bowl, add milk (3/4 cup) and bread (4 slices) and allow to sit for 10 minutes. Mash.
Add ground beef (2 pounds), grated onion (3 tablespoons), Parmesan (1/2 cup), eggs (2), Worcestershire sauce (1 tablespoon), garlic (2 teaspoons), salt (1/2 teaspoon), black pepper (1/2 teaspoon), and 1 tablespoon dry ranch dressing mix and combine well.
Using a medium cookie scoop, scoop out heaping scoops. Roll, and using gentle but firm pressure, compact the meatballs. You'll have 20-23 meatballs.
Arrange meatballs on prepared baking sheets. Place in preheated oven and bake for about 20 minutes, or until well browned.
Meanwhile, add beef broth (4 cups), the remaining dry ranch dressing mix, and Au Jus gravy (1 envelope) mix to a 6-quart slow cooker. Whisk to combine.
Transfer browned meatballs to the crockpot. Gently toss the meatballs to coat in broth. Arrange butter slices (1/2 cup) and pepperoncini peppers (8 whole) over meatballs.
Cover and cook on LOW, stirring halfway through the cooking time, until meatballs are cooked through, about 2 hours. Stir in pepperoncini juice (1 tablespoon).
Transfer meatballs and pepperoncini from crockpot to a platter AND cover with foil to keep warm. Pour liquid from crockpot into a saucepan. Over medium heat, simmer the liquid.
In a small bowl, whisk cold water (1/4 cup) with cornstarch (3 tablespoons). Gradually stir into the saucepan, continuing to simmer while gently stirring continuously, to activate the thickening process. (Don't overcook or stir/whisk too vigorously or the mixture can break down cornstarch and it won't thicken.) Simmer until you achieve the desired thickness. If you want the sauce thicker, you can continue to simmer and allow some of the liquid to evaporate or you can make more cornstarch slurry and repeat the process.
Serve over mashed potatoes or buttered noodles.