White Bean-and-Leek Soup

Two 1-inch thick slices of rustic white bread, cut into 1-inch dice (2 1/2 cups)
1 tablespoon extra-virgin olive oil
Kosher salt and freshly ground pepper
4 tablespoons unsalted butter
4 leeks (2 pounds), white and light green parts only, halved lengthwise and thinly sliced
2 thyme sprigs
Two 8 1/2-ounce cans cannellini beans, drained and rinsed
6 cups low-sodium chicken broth
1/4 cup freshly grated Parmigiano-Reggiano cheese, plus more for serving

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.