Preheat the oven to 190°C.
Cut the cauliflower into thin steaks, the onion into thin wedges. Arrange them in a roasting tray. You can add a few pieces of cauliflower leaves, if liked. Tuck the garlic cloves between them. Spray with oil, scatter the caraway seeds on top.
Roast for about 25 minutes.
Meanwhile, peel the turnips and cut them into small cube.
When the cauliflower & onion are tender, transfer them to a saucepan together with the cauliflower leaves (if not burnt) and any roasting jus.
Add the chopped turnip, then squeeze out the garlic flesh and this to the veggies too.
Pour in the stock, then bring to the boil. Cook for 5-10 minutes, or until the veggies are soft.
Using a hand-held blender purée the soup until smooth and silky.
Stir in the cider vinegar and serve.