Peel and devein the prawns (keeping the shells), leaving the tails on four of them, if you like. Butterfly these four and chop up the rest. Put the butterflied prawns in one side of a bowl, the chopped prawns in the other side, then cover the bowl and keep it in the fridge.
Put the prawn shells and half the oil into a pan and cook on high for about a minute, stirring a few times, until the shells turn pink. Add the garlic and, when you can smell it, add the passata. Sauté for a few more minutes. Add 1.25 litres(44 fl oz/5 cups) of hot water, the parsley stalks and lemon slices. Season with salt and pepper and bring to the boil. Simmer for about 20 minutes, then strain into a clean pan and keep warm over very low heat.
Heat the butter and the remaining oil in a heavy-based pan suitable for making risotto. Add the shallots and sauté for a few minutes over medium heat until lightly golden and softened, then add the rice. Stir and turn it for a few minutes so that it is well coated. Add a ladleful of the broth and stir with a wooden spoon until the liquid has been absorbed. Reduce the heat to low, add another ladleful of broth and stir until it has been absorbed. Carry on like this for about 40 minutes, then add the chopped prawns, chopped parsley and spinach. Continue cooking in the same way for another 10 minutes or so, until the rice is tender but still a little bit firm in the centre. If you run out of broth before this time, just carry on with hot water. Add the extra butter, sit the whole prawns on top of the rice and cook for a few minutes more until the prawns are pink. Sprinkle a little salt over the prawns, put a cloth over the rice, remove from the heat and leave to stand for 10 minutes before serving.