1 1/3 pounds boneless, skinless chicken breast halves (about 2 large)
Coarse salt and freshly ground pepper
5 tablespoons extra-virgin olive oil
1 leek, white and pale-green parts only, coarsely chopped and rinsed well
1 celery stalk, cut into 1/2-inch dice
10 ounces cremini mushrooms, halved if large
3 tablespoons all-purpose flour
3 tablespoons dry sherry
2 1/4 cups homemade or store-bought low-sodium chicken stock
3/4 cup whole milk
1 dried bay leaf
8 slices dense multigrain bread, crusts removed, slices cut into triangles
2 tablespoons chopped fresh flat-leaf parsley
1/3 cup finely grated Parmesan cheese (1 ounce)