Layer onion and potato in bottom of slow cooker. Add wine. Place turkey breast on top. Sprinkle turkey with onion soup, sage and thyme. Drizzle with butter. Cover and cook on low 6-8 hours or high 3-4 hours.
Optional: drain juices into a saucepan. Whisk in a little flour or cornstarch to thicken. Serve with meat and vegetables.