Preheat the oven to 180°C fan.
Mix the tahini and all the spices in a bowl. Squeeze in the juice of half a lemon and drizzle in the olive oil. Mix together and loosen with a little cold water if you need to. You want to be able to brush this mix onto the cauliflower. Season with salt and pepper.
Trim off the outer leaves of the cauliflower and cut the base so it can sit flat. Turn it upside down and pour a little of the spice mix into the gaps between the florets. Next get a basting brush and work the rest of the spice mix into the cauliflower, starting with the underneath and then turning it over.
Once the cauliflower is fully coated place in an over proof dish or tray and bake for aproximately one hour. Time will depend on how soft you like it, and how big your cauliflower is.
Meanwhile finely slice the red onion and put in a bowl with a sprinkle of salt and squeeze over the other half of the lemon juice. Set aside until ready to serve.
Lightly crush the pistachios in a pestle and mortar. Pick and roughly chop the parsley leaves.
When the cauliflower is cooked transfer it to a serving dish. Top with the onions, parsley and pistachios and drizzle over some olive oil.