Step 1 Soak the beans in water overnight. Drain.
Step 2 Heat the butter in a large, heavy-bottomed stockpot until melted. Add the carrots, leeks and celery and saute over medium-low heat 10 minutes, stirring occasionally, until tender but not browned.
Step 3Stir in one-half teaspoon thyme. Add the beans to the pot along with the chicken broth. Bring to a boil. Reduce the heat to a simmer. Simmer, partially covered, until the beans are tender, about 2 hours. Add additional water while cooking, if necessary, to keep beans covered. Season with salt and pepper.
Step 4Puree the soup in a food processor in batches until smooth, then return it to the pot and add the cheese. Heat the soup over low heat, stirring, until the cheese is just melted. Stir in the whipping cream, then remove from heat.
Step 5Adjust the seasoning if necessary. Spoon into bowls to serve. Garnish each serving with a sprinkle of the remaining thyme.