Heat the oven to 190C/fan 170C/gas 5. Toss the cauliflower florets and red onion slices with the turmeric and 2 tbsp olive oil. Season well and toss again, then spread on a large non-stick baking tray and cook for 25-30 minutes (add the leaves for the final 15 minutes) or until the cauliflower is tender and charred a little at the edges. Cool.