Blanch the leeks in boiling, salted water for 2-3 minutes until just tender, drain.
Heat the ratatouille in a medium saucepan with half a can of water until just boiling. Add the orzo, half the Walnuts and the basil, season and transfer to a 2 litre serving dish and bake for 15 minutes.
Stir in the ham and nestle in the leeks. Scatter over the cheese and remaining Walnuts and bake for a further 10 minutes.
