Bring 1litre/13⁄4pints water to the boil in a large pan, add 1 tbsp salt, then tip in the prawns, garlic and bay and return to the boil for 3-5 minutes until cooked, depending on the size of the prawns. Cool then shell the prawns. If they are large, you may wish to slice them, but leave smaller ones whole. Return the heads and shells to the stock and continue to boil for 20 minutes.
Put a medium size, heavy bottomed pan with a tight fitting lid on a low heat. Tip in the olive oil and 25g/1oz of the butter, and cook the shallots or onion for 5 minutes until soft and translucent. Add 1 tbsp of stock to stop them colouring, if necessary.
Stir in the rice so it absorbs all the buttery oil. When it is opaque and rustling as you stir, pour in all the wine. Keep stirring while the rice absorbs the wine – it should not evaporate into the air, so keep the heat low.
Meanwhile, strain the stock, return to the pan, top up with water and turn up the heat. Turn up the heat under the rice, too, as you need a moderate to lively heat. Stir the rice and add stock as needed to keep a veil of water above the level of rice for 10 minutes.
Add the prawns and tomato purée. From this point on I start to add less and less stock every time, to avoid the risk of forcing the rice to absorb too much stock. Start to taste the odd grain of rice. You want to stop cooking when it still has a tiny crunchy uncooked bit in the centre.
Season the risotto with salt and white pepper. Take the pan off the heat, add the remaining butter and enough stock to cover the risotto by about 2.5-5cm/1-2in, you have to judge how much it will absorb. Stir well, cover and leave it to rest for 10 minutes. Stir in the chopped parsley and serve at once.