In a large stockpot of dutch oven, melt the butter over medium heat. Add the leeks and saute until the leeks are bright green. If the leeks are beginning to brown, reduce the heat. Add the thyme, chicken stock, salt, white pepper, rice and chicken. Bring the mixture to a boil and reduce to a simmer. Simmer for 15 minutes or until the rice is tender. Add the roasted chicken and prunes and cook for another 5 minutes. Taste the soup. You may need to add more salt, pepper or lemon juice. NOTE: Adding lemon juice isn't traditional, but I find the little bit of acid really brightens the flavor of the soup so you may omit it if you wish.