Preheat the oven to 200oC/gas 6

In a saucepan, melt the butter then add the flour stirring together to form a paste. Gradually add the milk, whisking until smooth.

Cook gently for 5-7 minutes until thickened then add the mustard and season well. Remove from the heat and add in 50g of the cheddar, stirring until melted.

Tumble the cauliflower into a buttered gratin dish and pour over the sauce. Mix the rest of the grated cheese with the breadcrumbs if using and sprinkle over the top.

Bake in the centre of the oven for 40-45 minutes until bubbling and golden.