How to Make It
Preheat the oven to 350°. On a rimmed baking sheet, toss the bread cubes with the olive oil and season with salt and pepper. Toast until golden brown, about 10 minutes. Set aside to cool.
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.