Preheat the oven to 375°F (190°C). Line a baking sheet with foil and lightly spray or oil it.
In a large bowl, combine the ground beef, Italian sausage, bread crumbs, milk, and egg. Add the finely chopped red onion, 1 tablespoon minced garlic, parsley, 1 teaspoon salt, and Parmesan; mix gently until just combined.
Form the mixture into approximately 1-inch meatballs and place them on the prepared baking sheet, spaced slightly apart.
Bake the meatballs for 25 minutes, 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 the beef broth, tomato sauce, diced tomatoes, tomato paste, sugar, 2 teaspoons salt, dried basil, and Worcestershire sauce to the pot; stir to combine and bring to a simmer.
Transfer the baked meatballs into the sauce, bring the mixture back to a simmer, then reduce heat to low and simmer gently for 20 minutes, until the sauce thickens and flavors meld.
Meanwhile, cook the spaghetti according to package directions until al dente; drain.
Serve the meatballs and sauce over the cooked spaghetti.