Meatball-Stuffed Garlic Bread Sliders

Name: Meatball-Stuffed Garlic Bread Sliders
Prep time: 25 min
Cook time: 40 min
Total time: 1 h 5 min
Cuisine: American
Ingredients: 2 tablespoons olive oil, divided; 1 yellow onion, diced; 5 cloves garlic, minced, divided; 1 pound ground beef; 2 tablespoons chopped Italian parsley, plus more for garnish; 1 large egg; 0.25 cup plain bread crumbs; 1 cup shredded fontina cheese, divided; 2 teaspoons kosher salt; freshly ground black pepper to taste; 1 pinch cayenne pepper, or to taste; 3 tablespoons unsalted butter; 18 dinner rolls; 2 cups spicy tomato sauce; 2 tablespoons freshly grated Parmigiano-Reggiano cheese