Make the marinade by combining all the ingredients with 1½ tsp sea salt in a mixing bowl and stirring until well combined. Remove any leaves from the cauliflower and slice them lengthways. Put the cauliflower leaf slices in a bowl with a pinch of salt. Set aside for later.
Cut the cauliflower into thick slices about 1½cm-2cm thick – don’t worry if some of the florets break off, it’s all being roasted. Oil a large roasting tray and tip in the cauliflower slices and florets. Pour over the marinade and stir to coat, then cover with foil and marinate for at least 2 hrs, but you could leave it in the fridge all day or overnight if you wanted.
Heat the oven to 200C/180C fan/gas 6. Roast the cauliflower covered with foil for 40-45 mins. Meanwhile, make the sauce. Heat the olive oil with the cumin seeds in a heavy-bottomed frying pan over a medium heat. Add the onion and a pinch of salt and fry, stirring, for 8-10 mins until starting to caramelise. Tip in the chopped tomatoes and some seasoning. Turn the heat up slightly and cook, continuing to stir, for 6-8 mins until reduced, then add in the garlic, harissa and butter, and stir to combine. Blitz in a food processor until smooth, then pour back into the pan, add the chickpeas and cauliflower leaves, and warm through for a minute or two.
Remove the foil from the cauliflower and return the tray to the oven for 10 mins to caramelise. Stir the tahini and olive oil into the yogurt in a bowl and season. Once the cauliflower is ready, divide the warm sauce between plates, then top with the cauliflower slices and tahini yogurt. Scatter over the almonds. Serve with a well-dressed salad of crunchy leaves.