Cook the rice in double its volume of fast boiling salted water. Refresh it and dress it with 3 tablespoons lemon juice, 1 tablespoon oil and plenty of salt and pepper. Stir in the nuts and 5-6 tablespoons chopped mint or watercress. (Watercress is good but mint is even better for this dish.) Press the mixture firmly into a lightly oiled ring mould, cover and chill for a couple of hours or more. Close to serving time, halve the grapes and toss them in the juice of half a lemon plus scant 1 tablespoon oil. Add some mint or watercress sprigs. Skin, bone and flake the fish and mix it in gently. Unmould the ring of rice, put some of the fish and grape mixture into the centre and garnish with sprigs of mint or watercress. Serve the rest of the fish mixture in a separate bowl, and hand round the well-chilled yoghurt in a second bowl.