To cook cauliflower, place florets in a pan of boiling lightly salted water for 4 to 6 minutes. The cauliflower should be tender but not overcooked. Drain and keep warm.
Meanwhile to make sauce, melt butter in pan and add flour. Stir with a wooden spoon and it will form a paste.
Gradually add the milk - this is easier using a balloon whisk but a wooden spoon will do - and keep stirring to ensure it is smooth.
Bring to the boil to thicken then remove from heat.
Place cauliflower/ broccoli in a heatproof dish and pour over sauce.
Pre heat grill.
Mix together the cheese and crumbs and spoon over the sauce.
Grill for a few minutes until the crumbs are toasted and the cheese bubbling.