Remove the cauliflower’s tough outer leaves, but hold onto a few of the softer inner leaves. Roughly chop the cauliflower into florets and finely slice the thinner reserved leaves. Place the florets and sliced leaves onto a baking tray, in an even layer, then drizzle with some oil and scatter over all of the sumac. Transfer to the oven and cook for 45-60 minutes until the cauliflower is lightly browned all over.
Just before the cauliflower has finished cooking in the oven, roughly chop the onion and add to a large, heavy-based saucepan with the butter and a generous drizzle of oil. Cook over medium-low heat for 5-10 minutes until the onions are soft and translucent. Add the garlic alongside the cinnamon, cumin, allspice, nutmeg, cardamom, and chilli flakes. Increase the heat slightly and cook for a minute, stirring often.
At this stage, add the browned cauliflower and leaves to the pan and continue to stir for another minute or so. Season with salt and white pepper, then slowly pour the stock into the pan. Bring to a boil, then reduce the heat to a gentle simmer. Simmer the soup for 20 minutes, stirring occasionally.
Once cooked, allow to cool slightly then blend until smooth.
Return to a clean pan and add one tablespoon of the tahini and half of the lemon juice (being careful to catch any seeds) and gently reheat. Stir to combine and taste for seasoning. Add more salt or pepper if necessary.
While the soup reheats, in a small bowl whisk a tablespoon of tahini with two tablespoons cold water and the remaining lemon juice until smooth and pourable.
Divide the soup between shallow bowls, then drizzle over some of the whisked tahini, plus a dash of pomegranate molasses and a pinch of toasted pine nuts.