Preheat the oven to 375°F (190°C). Line a baking sheet with aluminum foil and spray or lightly oil it.
In a large bowl, combine the ground beef, Italian sausage, bread crumbs, milk, and egg; mix gently until just combined.
Add the finely chopped red onion, chopped parsley, 1 tablespoon minced garlic, 1 teaspoon salt, and 2 tablespoons grated Parmesan to the meat mixture and mix until evenly incorporated without overworking.
Scoop tablespoon-sized (about 1-inch) portions of the mixture and form into meatballs; place them on the prepared baking sheet.
Bake the meatballs 25 minutes, or until browned and cooked through.
While the meatballs bake, heat 1 tablespoon olive oil in a large skillet or pot over medium heat. Add 2 teaspoons minced garlic and cook until fragrant, about 30 seconds.
Add 4 cups beef broth, 15 ounces tomato sauce, 14 ounces diced tomatoes, 6 ounces tomato paste, 2 teaspoons sugar, 2 teaspoons salt, 2 teaspoons dried basil, and 1 teaspoon Worcestershire sauce; stir to combine and bring to a simmer.
Carefully add the baked meatballs to the simmering sauce, reduce heat to low, and simmer uncovered for 20 minutes until the sauce thickens and the flavors meld.
While the sauce simmers, cook the spaghetti according to package instructions until al dente, then drain.
Serve the meatballs and sauce hot over the cooked spaghetti.
