Bring a big pot of salted water to a boil for the spaghetti.
Heat 3 Tbl. of oil in an ovenproof skillet over medium heat
Add the onion, garlic and parsley and cook until the vegetables are soft but not browned, about 10 minutes.
Take the pan off heat and let cool.
Pour the milk over the bread in a bowl and let it soak while the vegetables are cooling.
Combine the meats in a large bowl.
Add the egg, parmesan cheese and season generously with the salt and pepper.
Use your hands to squeeze the excess milk out of the bread then add the bread to the bowl along with the cooled onion mixture.
Gently combine all the ingredients with your hands until just mixed; don't over work mixture.
Shape into 10 meatballs
Preheat the oven to 350º. Heat 2 tablespoons of oil in the skillet over medium heat and brown the meatballs on all sides, about 10 minutes.
Transfer then to a baking dish and spoon about half of the tomato sauce over.
Spread the mozzarella and scatter half of the basil on top
Drizzle with another tablespoon of olive oil.
Put the meatballs in the oven and bake until cooked through, about 30 minutes.
Meanwhile, cook the spaghetti in the boiling water until al dente.
Ladle on the rest the sauce and toss.
Spoon the meatballs onto the spaghetti and garnish with the rest of the basil.