In a large bowl, combine the turkey, minced onion, bread crumbs, Parmesan, parsley, milk, egg, garlic, pepper and 2 teaspoons of salt and mix well. Form the mixture into 16 meatballs
Put the marinara sauce in a slow cooker. Nestle the meatballs, the onion half and the basil sprig in the sauce. Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours, until the meatballs are cooked through and tender. Discard the onion half and basil sprig.
Bring a large pot of water to a boil, and add a small handful of salt. Cook the spaghetti in the boiling water according to package directions until al dente, 10 to 12 minutes. Drain the pasta and transfer to shallow bowls or a serving platter. Spoon the meatballs and sauce on the spaghetti.