Prepare the cauliflower – Bring a large pan of salted water to the boil, add the cauliflower florets, and cook for 4–5 minutes until just tender. Drain well and set aside.

Make the roux – Melt the butter in a saucepan over medium heat, add the flour, and cook gently for 1 minute, stirring constantly.

Add the milk gradually – Pour in the milk a little at a time, whisking continuously until smooth and thickened.

Enrich the sauce – Stir in the mustard, season with salt and black pepper, then add most of the grated cheese, mixing until melted and smooth.

Assemble the dish – Place the cauliflower into an ovenproof dish and pour the cheese sauce evenly over the top.

Finish with cheese – Sprinkle the remaining cheese over the surface.

Bake to perfection – Bake in a preheated oven at 200°C (180°C fan) for 20–25 minutes until bubbling and lightly golden.

Rest briefly – Allow to stand for a few minutes before serving so the sauce settles.