In a large saucepan, melt the butter. Add the leeks and thyme and cook over moderate heat, stirring frequently, until the leeks are tender, about 10 minutes. Add the beans and chicken broth, cover and simmer until the leeks are very tender, about 10 minutes. Remove from the heat. Stir in the 1/4 cup of Parmigiano-Reggiano and season the soup with salt and pepper. Ladle into bowls, garnish with croutons and pass additional cheese at the table.