Preheat oven to 250°C/480°F. Place the cauliflower florets into a bowl with 2 tablespoon olive oil and a generous pinch of salt. Toss to combine, then spread on a baking sheet and put into the oven to roast for 20-30 mins, until the edges of the cauliflower are beginning to blacken.
Meanwhile melt together the butter and the rest of the olive oil in a saucepan. Add the rosemary sprig and preserved lemon and fry very gently until fragrant. Add the onion, bay leaf, and chilli flakes, and fry until the onion is translucent, about 8 mins.
Add the potato, roasted cauliflower, stock, and miso, bring to a boil, and simmer for about 10 mins or until the potato is cooked. Blend until smooth. Serve with a swirl of chermoula, harissa, or other chilli sauce and sprinkled with nigella seeds.
Nutrition information is an estimate and will vary depending on the exact ingredients used.