Preheat the oven to 180°C.
Poke the clove through the bayleaf into the onion. Place this in a pan, pour over the milk and slowly heat it up, until simmering point. Turn off the heat and let it infuse for 10 minutes. Discard the onion.
Meanwhile, break the cauliflower into florets, then put them in a salted boiling water for 5 minutes or so, until just cooked through. Drain, rinse under cold water and drain again.
In another pan, melt the butter, add the flour, stir at very low temperature for about 3 minutes until it turns light. Add the milk, one ladle at a time, stirring constantly. Cook for 5-10 minutes until you get a silky, smooth sauce. Add two thirds of the cheese, then the cream and the mustard. Mix well. Season with salt & pepper - you can add a pinch of nutmeg, if liked. Combine gently.
Grease an ovenproof dish with butter and arrange the cauliflower pieces neatly side-by-side. Rub a little butter on top of them, then pour over the cheese sauce.
Scatter the remaining cheese on top and cook for 20 minutes until golden.