Bring a large saucepan of water to the boil, then add the chopped cauliflower, chopped broccoli and cubed carrots and cook for 5 mins – lift out a piece to test, it should be cooked. Drain the veggies then tip into an ovenproof dish.
Put the saucepan back on the heat and add the milk, flour and butter. Keep whisking fast as the butter melts and the mixture comes to the boil – the flour will disappear and the sauce will begin to thicken. Whisk for 2 mins while the sauce bubbles and becomes nice and thick. Turn off the heat, stir in 1 cup of cheese and pour over the veggies. Scatter over the remaining 1/2 cup of cheese and 3 tbsp breadcrumbs.