Bring a pan of salted water to the boil and have a bowl of iced water ready. Add the cauliflower to the pan, return to the boil and cook for 30 seconds. Drain the cauliflower, then plunge into the ice water. Once cold, drain well.
Keep 3 tbsp of the onion vinegar aside, then mix the cauliflower with the onions and the rest of the vinegar from the jar in a large bowl. Cover and steep in the fridge for at least 2 hrs, or overnight if possible.
To make the dressing, heat 1 tbsp oil in a pan, then add the garlic and ginger and cook for 1 min, stirring well. Stir in the turmeric and cook for 30 seconds, then remove from the heat. Add the mustard, 1 tbsp oil and the 3 tbsp pickling juice you reserved earlier. Cool for a few mins, then stir in the yogurt and season to taste.
Heat the oven to 180C/160C fan/ gas 4. Tip the kale onto a large baking tray. Season and rub 1 tbsp oil all over the leaves. Bake for about 6-8 mins till turning crisp at the edges. Once cooled, the kale will crisp up all over. Just before serving, drain the cauliflower and onion from the pickling juices and toss into the dressing, then fold the kale gently into it. Serve alongside our chicken terrine with leeks & apricots.