preheat oven to 200?

chop cauliflower into small florets

chop up the leeks

microwave cauliflower and leeks for 6-8 minutes till soft

melt butter in pot, stir in flour, till no lumps

add chicken stock and dry mustard,stir till it dissolves

add in milk, slowly stirring in

lay the cauliflower and leeks in baking tray,pour the white sauce over the vegetables, then cover with cheese

bake for 25mins or till cheese is crusty