1. In a small bowl, mix the milk and bread crumbs together with a fork. When the bread has softened, squeeze out most of the milk with your hands. Discard the milk.
2. Heat the olive oil in a large skillet over medium heat. Add the onion and red pepper and season with a light pinch of salt and pepper. Cook until the vegetables soften, about 5 or 6 minutes, and set aside to cool.
3. Combine the beef, turkey, bread crumbs, onion-pepper mixture, egg, 2 tablespoons parsley, nutmeg and 1 teaspoon salt in a medium-size bowl . Work the mixture with your hands unil it has an even consistency. With wet hands, shape the mixture into 12 meatballs. (This can be done 1 day in advance and stored in the refrigerator in one layer, tightly wrapped, until you are ready to cook them. The meatballs can be cooked in the time it takes to boil the spaghetti.)
For the sauce and spaghetti:
1. Heat the olive oil in a large skillet over medium heat. Add the onion and cook. When the onion begins to sizzle, add the meatballs, shaking the pan to keep them from sticking. Using tongs or a wooden spoon, gently turn and toss the onions and meatballs so they brown lightly.
2. Add the garlic and cook for 2 minutes more. Add the oregano and tomato sauce and the remaining 2 tablespoons parsley. Season with salt and pepper to taste. Simmer gently, uncovered, stirring the meatballs to coat them with sauce. Cook until the meatballs are cooked through. Keep warm.
3. Bring a large pot of well salted water to a boil. Add the spaghetti and cook until it is al dente, about 8 to 10 minutes. Drain the spaghetti and place it in a warmed pasta bowl. Pour the meatballs and sauce over the pasta. Serve with Parmesan cheese.