In a large bowl, combine ground turkey, breadcrumbs, Parmesan cheese, egg, minced garlic, chopped onion, parsley, salt, and black pepper. Mix until just combined.
Form the mixture into 16 evenly sized meatballs, about 1 1/2 inches in diameter.
Heat olive oil in a large skillet over medium heat. Add the meatballs in batches, cooking until browned on all sides, about 5 minutes per batch. Remove meatballs and set aside.
Pour marinara sauce into the skillet and bring to a simmer over medium heat.
Return the meatballs to the skillet with the sauce. Cover and simmer for 15 minutes, until meatballs are cooked through.
While the meatballs simmer, bring a large pot of salted water to a boil. Add spaghetti and cook according to package instructions until al dente, about 8-10 minutes.
Drain spaghetti and divide among four plates.
Top spaghetti with turkey meatballs and sauce. Garnish with chopped fresh basil and additional Parmesan cheese if desired.