Pre-heat the oven to 220 degrees C/425 degrees F. Toss the cauliflower in about a tablespoon of olive oil and some salt and pepper and roast for about 15 minutes in a single layer until almost tender.
Meanwhile, fry the pancetta in a deep pan for a few minutes in the rest of the olive oil, then remove from the pan and set aside.
Add the butter to the pan, then add the potato, onion, the leaves from the thyme stalks and the bay leaf and cook, stirring regularly, for 10 minutes. Add the garlic a couple of minutes before the end.
Add most of the cauliflower (reserve a little to garnish at the end) and stock, bring to the boil then simmer on a medium heat for 10 to 15 minutes.
Throw away the bay leaf, then add about half of the pancetta and blend the soup in batches until smooth. Leave some of it a bit chunkier if you like. Then return to the pan and reheat a little. Add the rest of the pancetta, reserving just a little for your garnish, and salt and pepper to taste.
Serve in bowls garnished with the few reserved cauliflower sprigs and the rest of the thyme and pancetta. Swirl in the Greek yoghurt just before serving if desired.
