Preheat the oven to 240°C (475°F/Gas 8). Drizzle the onion with a little oil, season with sea salt and roast for about 25 minutes, or until soft and caramelized.
Tear the witlof leaves in half, then tear the radicchio leaves into similar sized pieces. Quarter each pear
and remove their cores. Cut each quarter into thirds lengthways and drizzle with the juice of half a lemon to stop the pears from browning.
Place the leaves in a bowl and drizzle with oil and the juice of 1 lemon, add some sea salt and give a good grind of pepper.
Divide the leaves among four large plates and then scatter the roast onion, pear, Roquefort and walnuts over the leaves. Drizzle each salad with a little extra oil and squeeze some lemon juice on, as well. Give a last grind of pepper and serve.