Preheat the oven to 200°C (180°C fan). Lightly grease a baking dish.
Bring a large pot of salted water to a boil and cook the cauliflower florets for 5 minutes until just tender. Drain well and set aside.
Melt the butter in a saucepan over medium heat, then stir in the flour and cook for 1 minute.
Gradually whisk in the milk, stirring constantly until the sauce thickens and becomes smooth.
Remove from heat and stir in the cheddar cheese, Dijon mustard, salt, and black pepper until melted.
Place the cauliflower into the prepared baking dish and pour the cheese sauce evenly over the top.
Sprinkle with breadcrumbs and extra cheese, if using.
Bake for 20–25 minutes until golden and bubbling.
Sprinkle chopped parsley on top and serve hot.