Heat the oven to 425°F and oil a rimmed sheet. Soak breadcrumbs in milk for 3 minutes, then mix gently with turkey, sausage, egg, basil, meatball salt and garlic powder. Shape 24 small meatballs and bake for 16 to 20 minutes until the centers reach 165°F.
Meanwhile, heat olive oil in a saucepan and cook onion for 6 minutes. Stir in garlic for 30 seconds, then blend roasted peppers with crushed tomatoes until fairly smooth and pour into the pan with sauce salt; simmer for 5 minutes.
Transfer hot sauce and hot meatballs to a 5-quart slow cooker. Cover and cook on HIGH for 2 hours, turning gently once, until the meatballs remain at least 165°F and the sauce clings to them.
Near serving, toast the split rolls cut-side up on a sheet at 400°F for 4 to 5 minutes until the edges are lightly crisp.
Place 4 meatballs in each roll with a modest spoonful of sauce and divide Fontina over them. Broil 6 inches from the heat for 2 to 3 minutes until melted.
Rest for 2 minutes and serve one sub per person, spooning any extra sauce alongside only as needed.