Peel the prawns, reserving the heads and shells.
Melt 100 g (3½ oz) butter in a large frying pan and fry the prawn heads and shells for 3–4 minutes. Strain and return butter to the pan, discarding heads and shells.
Add the onion and garlic and cook for 5 minutes until softened but not coloured. Add the rice and stir well to coat the grains with the butter. Add the peas, then pour in the wine. Bring to the boil and cook, stirring, until reduced by half.
Add the hot stock, a large ladleful at a time, and cook, stirring constantly until each addition has been absorbed before adding the next. Continue in this way until all the stock has been absorbed and the rice is creamy but the grains are still firm. This should take about 20 minutes.
Melt the remaining butter in a separate frying pan, add the prawns and cook, stirring, for 3–4 minutes. Stir into the rice with the pan juices and mint and season to taste with salt and pepper.