Heat a pan of water until boiling. Add the cauliflower florets and cook for 5-6 minutes until they are just tender.
Drain and place in a buttered ovenproof dish.
Make the sauce by mixing some of the cold milk with the cornflour to make a thick paste, then heat the milk to just below boiling point, about 3 minutes in the microwave or heat in a pan on the stove.
Pour the hot milk into the cornflour paste and mustard and stir until thoroughly mixed, then heat for 1 more minute in the microwave until thickened.
Season the sauce with salt & pepper to taste and pour over the cauliflower.
To make the soup
Heat the sunflower oil in a large pan, add the chopped onions and celery and cook on a medium heat until the onion is soft and transparent and the celery has started to soften.
Add the stock and simmer gently for 10 minutes until the celery is soft.
Add the cauliflower in cheese sauce and cook for a further 10 minutes.
Take the pan off the heat and leave to cool for about 20 minutes.
Puree the soup with a stick blender or in a liquidiser/blender, adding some milk if the mixture becomes too thick.
Season to taste and garnish with parsley.
Notes
Make double the quantity of Cauliflower Cheese. Put half into a buttered ovenproof dish and bake at 180C for 20-30 minutes until the sauce is bubbling and the top is golden. Serve on its own with crusty bread for lunch or supper, or serve a side dish with roasts and stews.