Broccoli Cheese Casserole With Crackers

Name: Broccoli Cheese Casserole With Crackers
Ingredients: 2 heads broccoli (2 ½ pounds), cut into florets, stems peeled and cut into ½-inch pieces; Kosher salt and freshly ground pepper; 2 tablespoons unsalted butter, plus 2 tablespoons melted; 1 small onion, finely chopped (1 cup); 8 ounces cremini mushrooms, thinly sliced; 3 tablespoons unbleached all-purpose flour; 3 cups milk; 2 teaspoons Dijon mustard; 1 teaspoon Worcestershire sauce; ⅛ teaspoon cayenne pepper; ½ cup sour cream; 8 ounces shredded Gouda cheese (about 3 cups); 30 saltine crackers, crushed (2 ¼ cups)