Heat a large heavy-based saucepan and add the oil and half the butter. Heat until it is foaming then add the onion and gently cook for about 5 minutes until beginning to soften. Add the garlic and cook for a few minutes then stir in the rice and heat through for a minute until shiny and opaque.
Pour in the wine and boil for 1 minute to allow the alcohol to evaporate, stirring constantly. Turn down the heat to medium heat and begin to add the stock a ladleful at a time allowing the liquid to be absorbed into the rice before adding more.
After 15 minutes of cooking, add the frozen peas and cook for another 5-7 minutes.
When the rice feels soft and fluffy and the texture is creamy but each grain is still firm to the bite in the centre the risotto is ready. At this point take the risotto of the heat and stir in the remaining butter, cheese and mint and season to taste. Leave to rest with the lid on for a few minutes then serve immediately.