Preheat your oven to 180 C / 350F
Make a white sauce with the butter, flour, and milk (adding the garlic at the beginning) and whisk until it is smooth and the flour is cooked through. Season with salt, pepper, and a pinch of nutmeg.
Stir through the Dijon mustard and add the broccoli.
Continue to cook this for about 20 minutes and until the broccoli is softened and breaking apart (I cooked it with the lid and at a very low temperature)
Add the cheese and allow this to melt in.
Using a handheld blender process the broccoli and cheese sauce until smooth adding a little extra milk if necessary.
In an appropriately sized oven-proof dish, arrange the head of cauliflower that you have broken into large florets.
Pour over the cheese sauce.
Mix the olive oil with the bread crumbs and thyme leaves and scatter this over the top.
Bake in the oven for 1 hour until golden brown.
I added a small handful of grated parmesan about 10 minutes before the end. You could also add the almonds as Jamie does