This is comfort food, plain and simple. It's also a good way to get kids to eat their vegetables. The recipe calls for sharp cheddar as the basis for the cheese sauce, but other cheeses can be substituted, including Gruyére, Swiss, or Monterey Jack. Leftovers can be reheated in the microwave.
Position a rack in the middle of the oven and preheat oven to 425 degrees F.
Place the cauliflower florets in a steamer insert or basket. Steam over boiling water for 5 minutes. Drain and set aside in a large bowl.
Steam the broccoli in the same manner for 4 minutes. Plunge the broccoli into a bowl of ice water to stop the cooking process and retain its bright green color. Drain thoroughly, then add the broccoli to the cauliflower.
Heat 3 tablespoons of butter in a medium-size pan over medium-high heat. Add the flour and cook, stirring constantly, for 2 minutes. Add the milk all at once and whisk to combine. Cook until the mixture thickens, 3 to 5 minutes.
Add the cheese, Worcestershire, and mustard, and lower the heat to medium. Cook, stirring frequently, until the cheese melts. Season to taste with salt, pepper, and nutmeg.
Pour the cheese sauce into the bowl of cauliflower and broccoli and stir gently to combine. Transfer the mixture to a 2-quart baking pan. Sprinkle with the parmesan.
Melt the remaining 2 tablespoons of butter in a small pan over medium heat. Mix the breadcrumbs with the butter. Scatter the crumbs over the top of the casserole.
Bake until the topping turns golden and the cheese sauce bubbles, 15 to 20 minutes. Serve warm.