Preheat oven to 200C/390F
Lightly grease a large shallow casserole dish.
Break the cauliflower into florets and steam or boil for 3 - 5 minutes until just tender. Drain very thoroughly.
Melt the margarine in a large saucepan over a gentle heat.
Add the flour and cook gently for a couple of minutes.
Remove from the heat.
Add a little of the milk and stir or whisk to make a lump free paste.
Add the rest of the milk gradually, whisking and stirring with each addition until you have a smooth sauce.
Return to the heat and bring to a boil, stirring constantly.
Reduce the heat and simmer, stirring until you have a thick sauce. You may need a little more milk.
Add the nutritional yeast. mustard, and soy sauce. Taste and add more of each if needed (*see notes).
Add a pinch each of salt, pepper and nutmeg. Taste and adjust seasoning if needed.
Stir the cooked and well drained cauliflower into the sauce and spoon into your prepared casserole dish.
Bake in the preheated oven for about 30 minutes or until bubbling and browning.