1. 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.

2. 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.