Meatball-Stuffed Garlic Bread Sliders

18 dinner rolls (I used Hawaiian rolls, but will try regular less-sweet ones next time)
3 tablespoons butter infused with 3 or 4 cloves of crushed or minced garlic
1 pound grass-fed beef
1 diced yellow onion and a few cloves of garlic sautéed until soft in olive oil
2 tablespoons freshly chopped parsley, plus more for garnish
1 large egg
1/4 cup plain breadcrumbs
1 cup grated fontina cheese, divided (big handful in the meatball mix, and the rest on top)
2 teaspoons kosher salt
freshly ground black pepper and cayenne to taste
2 cups prepared tomato sauce
freshly grated Parmigiano-Reggiano cheese for the top