Place onion in a 3-1/2- or 4-quart slow cooker. Use a sharp knife to carefully cut chicken wings into three portions; discard wing tips. Place wing pieces over onions in cooker. Stir together the coconut milk, fish sauce, and curry paste. Pour over chicken wings.
Cover and cook on low-heat setting for 5 to 6 hours or on high-heat setting for 2-1/2 to 3 hours. Remove chicken from cooker with a slotted spoon; cover and set aside. Skim fat from cooking liquid. In a medium saucepan stir together the cornstarch and water; stir in the cooking liquid. Cook and stir over medium heat until thickened and bubbly. Cook and stir 2 minutes more. Stir in basil. Serve over wings. Makes 12 appetizer servings.