Heat a medium saucepan coated with cooking spray over medium-low heat. Add onion and thyme; cook 3 minutes, stirring frequently. Stir in radishes; cook 1 minute, stirring constantly. Add wine; bring to a boil. Cover, reduce heat, and simmer 5 minutes or just until radishes are tender.
Remove radishes from pan with a slotted spoon. Cook wine mixture 2 minutes or until slightly thick, stirring occasionally. Remove from heat; stir in butter and salt. Pour wine mixture over radishes, and sprinkle with parsley. Serve immediately.