Preheat oven to 190⁰ Gas 5.
Cook the leeks for about 7 mins. Then add the cauliflower, cook until tender then drain. Melt the butter and stir in the flour, and stir for a few seconds more. Remove from fire and gradually add the milk, stirring well after each addition. Return to heat and stir until thickened and smooth. Take off the fire stir in mustard and most of the cheese; adjust seasoning if necessary. Transfer vegetables into an oven-proof dish and pour over the cheese sauce. In a small bowl, mix together the crumb topping and scatter it on top along with the remaining cheese. Put the dish in the oven for about 15 mins. Until top is golden. Serve immediately.