White Bean and Mushroom Gratin

Name: White Bean and Mushroom Gratin
Ingredients: 1/2 cup extra-virgin olive oil , divided; 10 ounces cremini mushrooms , trimmed and sliced ½ inch thick; 3/4 teaspoon salt; 1/2 teaspoon pepper , divided; 4-5 slices country-style bread , cut into ½-inch cubes (5 cups); 1/4 cup minced fresh parsley , divided; 1 cup water; 1 tablespoon all-purpose flour; 1 small onion , chopped fine; 5 garlic cloves , minced; 1 tablespoon tomato paste; 1 1/2 teaspoons minced fresh thyme; 1/3 cup dry sherry; 2 (15-ounce) cans great Northern beans; 3 carrots , peeled, halved lengthwise, and cut into ¾-inch pieces