Mushroom and Leek Gratin

Name: Mushroom and Leek Gratin
Ingredients: 5 tablespoons unsalted butter , divided; 1⁄3 cup panko bread crumbs; 1½ teaspoons table salt , divided; 4-5 pounds leeks , white and light-green parts only, halved lengthwise, sliced 1 inch thick, and washed thoroughly (10 cups); 1 tablespoon water; 10 ounces cremini mushrooms , trimmed and quartered (halved if small); 10 ounces shiitake mushrooms , stemmed and sliced thin; ½ teaspoon pepper; 1 tablespoon all-purpose flour; 2 garlic cloves , minced; 2 teaspoons minced fresh thyme; 1 cup heavy cream; ¾ cup chicken broth; 4½ ounces Parmesan cheese , grated (2¼ cups)