In a large saucepan, combine the butter, leeks, carrots, onions, celery, and spices. Cover with water and simmer until the vegetables are tender. When vegetables are tender, place ¾ of the contents of the saucepan into a blender and puree until creamy. Pour contents back into the saucepan. Add cream, chopped tomatoes, mushrooms, and watercress. Season to taste with salt, black pepper, and white pepper. Simmer for 10 more minutes. Serve with french crusty bread and enjoy!