Martha’s Broccoli Casserole

INGREDIENTS

2 heads broccoli (2 1/2 pounds), cut into florets, stems peeled and cut into 1/2-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
1/8 teaspoon cayenne pepper
1/2 cup sour cream
8 ounces shredded Gouda cheese (about 3 cups)
30 saltine crackers, crushed (2 1/4 cups)

Preheat oven to 400 degrees. In two batches, cook broccoli in a large pot of salted boiling water until slightly tender, about 3 minutes. Drain and rinse with cold water to cool. Drain well.
Melt 2 tablespoons butter in a large straight-sided skillet over medium-high. Add onion and mushrooms; season with salt and pepper. Cook, stirring occasionally, until tender and starting to brown, about 7 minutes. Add flour and cook, stirring constantly, 1 minute. Stir in milk in a slow, steady stream. Cook, stirring constantly, until mixture coats the back of a spoon, about 9 minutes. Remove from heat. Stir in mustard, Worcestershire, cayenne, sour cream, and cheese. Season with salt and pepper. Stir in cooled broccoli. Transfer to a 2 1/2-quart shallow baking dish. Combine crackers and remaining 2 tablespoons melted butter. Sprinkle evenly over top of casserole. Bake until bubbling and top is browned, about 20 minutes.