Preheat the oven to 350 degrees F. Grease an 8- by 8-inch baking dish with 2 tablespoons of the butter.
Melt 2 tablespoons of the butter in a large pot over medium heat, and then add the mushrooms and shallots and saute until the mushrooms are golden brown, about 10 minutes. Add the flour and cook for 2 minutes, and then add the milk, mustard powder, nutmeg and cayenne pepper and sprinkle with salt and pepper. Cook until thick, about 5 minutes. Stir in the cheese and broccoli until the cheese is melted.
Pour the cheese, broccoli and mushroom mixture into the casserole dish. Melt the remaining butter and toss with the panko. Sprinkle evenly over the casserole. Sprinkle the duck prosciutto evenly on top. Sprinkle with more salt and pepper, and then bake in the center of the oven until bubbly around edges, and the panko and duck are crispy, 25 to 35 minutes.
Slice and serve on a plate or in a bowl, on its own or over fusilli.