Preheat broiler. If desired, use a sharp knife to carefully cut off tips of the wings; discard tips. Cut each wing at joint to make 2 pieces. Place wing pieces in a single layer on the unheated rack of a foil-lined broiler pan. Broil 4 to 5 inches from the heat for 10 to 12 minutes or until chicken is brown, turning once. Drain off fat.
Meanwhile, in a 3 1/2- or 4-quart slow cooker combine coconut milk, fish sauce, and curry paste. Add wing pieces and onion, stirring to coat with curry mixture.
Cover and cook on low-heat setting for 3 to 4 hours or on high-heat setting for 1 1/2 to 2 hours.
Using a slotted spoon, remove chicken from cooker; cover with foil to keep warm. Skim fat from curry mixture.
For sauce, in a medium saucepan combine cornstarch and water; stir in curry mixture. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Stir in basil, if desired. Serve chicken wings with sauce.