Preheat the oven to 375°F. Line a baking sheet with foil and spray or lightly 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, 1 teaspoon salt, chopped parsley, and grated Parmesan; mix gently until just combined.
Scoop or roll the mixture into roughly 1-inch meatballs and place them on the prepared baking sheet, leaving space between each.
Bake the meatballs for 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 the beef broth, tomato sauce, diced tomatoes, tomato paste, 2 teaspoons sugar, 2 teaspoons salt, dried basil, and Worcestershire sauce to the pot; stir to combine and bring to a simmer.
Carefully add the baked meatballs to the sauce, bring the sauce to a gentle simmer, then reduce heat to low and simmer for 20 minutes, until the sauce thickens and flavors meld.
While the sauce simmers, cook the spaghetti in a large pot of salted boiling water according to package directions until al dente; drain.
Serve the hot meatball sauce over the cooked spaghetti.