Put the milk in a small saucepan, add the flour and cook over a medium heat, whisking vigorously until the sauce has thickened and is smooth. Add salt and pepper, the mustard powder and nutmeg and reduce the heat to very low. Cook for 5-6 minutes, then remove from the heat and set aside. Heat the oven to 200°C.
Cook the cauliflower and broccoli in boiling, salted water for 3-4 minutes until just tender, then drain.
Meanwhile, cook the macaroni in plenty of boiling, salted water for 4-6 minutes until tender. Drain well, then return it to the pan.
Add the white sauce and Cheddar, then stir in the cauliflower and broccoli. Pour the mixture into a large ovenproof dish. Sprinkle over the breadcrumbs and Parmesan.
Cook in the oven for 10-15 minutes or until the top is puffy and lightly browned. Serve at once with a green salad.