Pre-heat oven to 200C/fan 180C/gas 6.
Melt the butter in a heavy oven proof pot, add the onions and cook for 3 minutes until softened then add the celery, garlic and chorizo and cook for another 5 minutes.
Add the rice and cook for 1 minute and then add the vegetable stock, put the lid on the pot and place in the oven and leave for 15 minutes.
Add the tomatoes and leave for 3 minutes.
Add the peas and prawns and leave for 2 minutes.
Remove from the oven, stir in the cheese and serve.