Line a large heavy saucepan with the outer leaves from the cauliflower. Put the cauliflower on top. Sprinkle with the onion, olives and salt, then pour over the olive oil. Cover with a lid, set over the lowest heat and cook gently for about 40 minutes, until very tender.
Carefully lift the cauliflower out onto a plate and pile the onions & green olives on top. Drizzle with more olive oil and serve.
Serves 4