1. Heat a large skillet over medium heat. Add oil to pan; swirl to coat. Stir in onion; cover and cook 10 minutes. Uncover; stir, and reduce heat to medium. Cook 15 minutes or until onion is tender, stirring frequently. Add 1/4 cup water, stirring to loosen browned bits; cook 10 minutes, stirring frequently. Remove from heat; cool 10 minutes.
2. Place marinara sauce in a Dutch oven. Bring to a simmer over low heat.
3. Combine onion mixture, basil, and next 6 ingredients (through egg) in a large bowl. Add ground sirloin, ground pork, and crumbled bread; stir gently to combine. Shape meat mixture into 18 (2-inch) balls. Place meatballs in sauce. Bring to a boil; cover, reduce heat, and simmer 30 minutes or until meatballs are done.
4. While meatballs simmer, cook pasta according to package directions, omitting salt and fat; drain.
5. Serve meatballs and sauce over pasta. Sprinkle with Parmesan cheese, if desired.