Strip the leaves from the cauliflower stems and roughly chop the stems into 2cm pieces. Steam or boil in boiling water for 2 minutes until the leaves are wilted and the stems are still a little crunchy. Drain and pour into an ovenproof dish.
Meanwhile, make the cheese sauce by melting the butter over a medium heat, then stir in the flour and cook for 1 minute before gradually adding the milk, stirring all the time.
Bring to a boil, then reduce the heat and simmer for 2 minutes until thickened.
Mix in the mustard and season with salt and pepper and finally add half the cheese and mix well.
Pour the sauce over the leaves and sprinkle over the remaining cheese and breadcrumbs.
Place under a preheated grill for 5 minutes until the cheese has melted and the breadcrumbs are golden.