Break the cauliflower head into florets, and add them to a pan with salted water.
Boil for 3-4 minutes until the cauliflower florets begin to soften slightly, but they still hold their shape well.
Remove from the heat, drain and set aside.
Cook the bacon lardons until crispy and golden brown, then remove from the pan and set aside.
In the same pan, melt the butter, then add the flour and whisk well.
Pour in the milk, and whisk until it thickens into a sauce.
Add the parmesan, half of the grated cheddar cheese, dijon mustard and black pepper.
Mix well.
Butter an ovenproof dish, arrange the cauliflower, top with bacon, and drizzle the bechamel sauce all over it.
Top with the remaining cheddar cheese.
Bake in the preheated oven at 200 degrees Celsius (390 Fahrenheit) until golden and bubbling hot.