Combine ground meats with egg, BelGioioso Romano, parsley, garlic, salt and pepper in mixing bowl. Blend in breadcrumbs. Slowly add water, mixing until moist. Shape meatball mixture into slider sized patties. Refrigerate patties until ready to cook.
Grill or pan-grill meatball burgers over medium heat on lightly oiled grill. Cook thoroughly, then add 1 slice of cheese atop each burger. Cook 1 minute longer to melt cheese. Toast buns, cut side down on grill. Serve each burger with 1/2 Tbsp. grilled onions and 1/2 oz. warm marinara sauce.