Persian rice (Tahdig) with courgette and pickled walnuts

Method

1. Rinse the rice under cold running water until it runs clear, transfer to a bowl, cover with cold water, add the salt and set aside for 30 minutes. Drain the rice, add to a large pan of lightly salted boiling water and cook for 5 minutes. Drain again, then rinse under cold running water. Set aside (chill if not using straightaway).
2. Light the barbecue and heat until the coals are glowing white. Set the casserole on the grill bars (or set it over a medium heat on the hob).
3. Lightly toast the walnut halves in the casserole, then set aside in a small bowl. Add a little butter and a glug of oil to the casserole and gently cook the shallots for 10 minutes (move the casserole slightly to the side) until softened. Add the garlic and continue to cook for a further minute, then scoop both out with a slotted spoon and add to the bowl with the toasted walnuts. Generously grate over some nutmeg. Stir in, then set aside (see Make Ahead).
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4. Wearing oven gloves, lay a sheet of non-stick baking paper over the base of the casserole so it comes a few centimetres up the sides. Add another glug of oil and the 50g butter to the casserole (on top of the paper) and let them melt together.
5. Transfer a third of the part-cooked rice into a separate bowl and mix with the yogurt. Spoon this mixture in an even layer over the melted butter in the casserole. Stir the walnut/shallot mix into the remaining rice and spoon it over
   the yogurt rice, piling it up slightly in the centre. Use the handle of a wooden spoon to poke about 6 holes right through the rice to the base of the casserole, which will allow the steam to rise.
6. Wrap the casserole lid with a clean tea towel (tie it on top) and set it over the casserole. Move it to the coolest part of the barbecue (or reduce the hob heat to as low as it can go) and cook for 30 minutes. Turn the casserole every 10 minutes to ensure the base cooks evenly. At the end of cooking time, turn out the rice onto a serving plate (wear heavy oven gloves to protect your hands) – it should be crispy-crunchy on the bottom (now the top!).
7. Meanwhile, for the topping, toss the courgettes with the oil and cook on the hot part of the barbecue (or in a hot griddle pan), turning often until tender – about 5 minutes.
8. Roughly chop the courgettes, then toss with the watercress and pickled walnuts and sprinkle over the turned-out rice. Crumble over the feta and sprinkle over the chilli flakes.
9. In a small bowl, mix the lemon zest, juice and saffron until the juice turns golden. Drizzle over the rice and vegetables and serve with extra yogurt on the side.