Drip Beef Meatballs

Name: Drip Beef Meatballs
Ingredients: 4 tablespoons (1/2 stick) salted butter; 2 large onions, thinly sliced; Kosher salt; 2 pounds ground chuck steak; 2/3 cup seasoned breadcrumbs; 2/3 cup grated mozzarella; 1/4 cup chopped fresh parsley; 1 tablespoon grainy Dijon mustard; 1 large egg, beaten; Kosher salt and freshly ground black pepper; 2 teaspoons cornstarch; 1 cup beef stock; One 16-ounce jar sliced pepperoncini, plus the liquid from the jar; One 2-ounce packet French onion soup mix; 3 cloves garlic, minced; 12 hotdog or brat buns