Broccoli and Wild Mushroom Casserole

Name: Broccoli and Wild Mushroom Casserole
Ingredients: 3/4 pound mixed wild mushrooms, such as cremini and shiitake, stemmed and quartered; 1 stick unsalted butter, plus 1 tablespoon melted; 1 large onion, minced; 4 large celery ribs, finely diced; 3 tablespoons all-purpose flour; 1 cup chicken stock or low-sodium broth; 1/4 cup milk; Salt and freshly ground pepper; 1 cup mayonnaise; 2 1/2 pounds broccoli—heads cut into 1-inch florets, stems peeled and cut into 1/2-inch dice; 1 1/2 cups coarsely shredded sharp cheddar cheese (6 ounces); 1 1/3 cups crumbled Ritz crackers (from 1 sleeve, about 35 crackers)