Preheat the oven to 220C/420F.
Spread the cauliflower out on a baking tray in one layer and drizzle with the olive oil. Roast for 15 to 20 minutes or until the cauliflower is golden and almost caramelised.
At the same time, boil the potato chunks until they are soft (about 15 minutes). Also warm the milk in a small saucepan with one of the thyme sprigs or the dried thyme.
Put aside a small handful of the roasted cauliflower florets. Then put the rest into the bowl of a food processor and blend with a bit of the warm milk (thyme sprig removed!).
Add the potato, butter, cheese and as much of the rest of the milk as you need to blend to a smooth puree.
Finally, grind in plenty of salt and pepper and blend for a couple more seconds.
Scrape the mash into a nice serving dish. Top with the reserved cauliflower florets, a sprinkle of fresh thyme leaves (if using fresh) and a drizzle of olive oil.
