Preheat your oven to 375°F (190°C).

Bring a large saucepan of salted water to a boil.

Add the cauliflower florets and cook for about 5 minutes until slightly tender, but not fully cooked.

Carefully drain the cauliflower in a colander and set it aside.

In the same saucepan, melt the butter over medium heat.

Stir in the flour and cook for a minute to create a roux.

Gradually whisk in the milk, stirring constantly until the mixture thickens.

Once thickened, remove from heat and stir in the shredded cheese until it melts and becomes smooth.

Return the cauliflower to the saucepan and mix well with the cheese sauce.

Season with mustard powder, salt, and pepper to taste.

Transfer the mixture to a baking dish and sprinkle with breadcrumbs if using.

Bake for 20 minutes or until the top is golden and bubbly.