Remove the shells and heads from the prawns and make a litre (1¾ pints) of stock by boiling the debris in a litre or so of water. Strain it into a clean saucepan, and stir in the rice. Bring to the boil, cover and lower the heat to the merest simmer. Cook for 18 to 20 minutes until the rice is cooked and the liquid absorbed. Add the finely grated lemon zest, a dash of juice and the parsley. Fluff it up carefully with two forks, cover and leave while you cook the shellfish. This is quickly and simply done, heating the oil and butter in a large frying pan or wok, stirring in the prawns, and cooking them until they turn pink. Pour on the spirits, stand well back and light. Shake the pan, distributing the juices, and when the flames have disappeared, season lightly.
Mix the harissa and mayonnaise. If you cannot find this brick-red chilli-hot paste from Tunisia, improvise by mixing into the mayonnaise, little by little, to taste, some cayenne, plenty of paprika, some dried crumbled oregano leaves and a little tomato paste.
To serve, border a large platter with the rice and spoon the prawns and the rest of the lemon juice into the centre.