Bring a large pot of salted water to a boil.
Add the cauliflower florets and blanch for 5 minutes until just tender. This helps soften the cauliflower while keeping its shape intact.
Drain the cauliflower well and set aside. Ensuring it is properly drained prevents the sauce from becoming watery.
In a medium saucepan, melt the butter over medium heat.
Stir in the flour and cook for 1–2 minutes, stirring constantly to form a roux.
Gradually whisk in the milk, ensuring there are no lumps. Continue stirring until the sauce thickens.
Add the mustard, salt, and pepper, followed by the grated cheeses. Stir until the cheese is completely melted and the sauce is smooth.
Preheat your oven to 200°C (180°C fan) / 400°F / Gas Mark 6.
Arrange the blanched cauliflower in an ovenproof dish, ensuring the florets are spread out evenly.
Pour the cheese sauce over the cauliflower, making sure it is fully covered.
Sprinkle extra cheese and breadcrumbs on top for an added crispy texture.
Place the dish in the oven and bake for 20–25 minutes, or until the top turns golden brown and bubbling.
Allow the dish to cool slightly before serving, as this helps the sauce thicken slightly.