Heat half the butter in a large saucepan over a medium heat. Add the onion and gently sauté for about 5 minutes until soft.
Add the rice to the onion and stir until all of the rice has been coated by the butter.
Add the stock in, a ladleful at a time, stirring frequently and waiting until all of the stock has been absorbed by the rice before adding another ladleful.
After about 10-15 minutes, add the peas; stir in well before continuing to add the stock.
Once all of the stock has been added, stir in the prawns.
Allow the prawns to heat up in the mixture for a minute or two before removing from the heat and adding in the remaining butter, along with a large handful of freshly grated parmesan.