Preheat the oven to 180°C, 160°C fan., Gas Mark 4.
Bring a large pan of water to the boil and cook the cauliflower and broccoli for 5-6 minutes, or until just cooked.
Drain the cauliflower and broccoli and allow to steam for a few minutes, before placing the cooked vegetables into a large ovenproof dish.
Using the same pan, on a gentle heat whisk the butter, flour and milk together until combined.
Increase the heat a little and add the mustard and ⅔ of the cheese. Stir with a wooden spoon for a few minutes on medium heat, or until the sauce has thickened.
Pour the sauce over the cauliflower and broccoli. Sprinkle over the remaining cheese and breadcrumbs.
Bake for 10-12 minutes, until brown and the sauce is bubbling.
Serve piping hot with a crunchy side salad.
