Creamy Lima Bean Gratin

Name: Creamy Lima Bean Gratin
Ingredients: 4 tablespoons unsalted butter, 2 tablespoons melted; 2 large leeks, white and tender green parts, thinly sliced crosswise; 3/4 pound shiitake or cremini mushrooms, stems discarded and caps thinly sliced; 3 cups chicken stock or low-sodium broth; 3 cups heavy cream; 2 bay leaves; 1 tablespoon finely chopped tarragon; Salt and freshly ground pepper; Four 10-ounce boxes of frozen baby lima beans; 1/2 cup fine, dry bread crumbs; 1/4 cup freshly grated Parmesan cheese