Heat olive oil and 25g of butter in a large heavy-based saucepan until foaming.
Sauté the onion for 5 minutes until softened, then add garlic and cook for a few minutes.
Stir in the rice and heat for 1 minute until it appears shiny and opaque.
Pour in 150ml of white wine and boil for 1 minute while stirring constantly.
Add the stock one ladleful at a time over medium heat, letting it absorb before adding more.
Add the frozen peas after 15 minutes of cooking and simmer for 5 to 7 minutes more.
Remove from heat when the rice is creamy but firm in the center.
Stir in the remaining butter, pecorino, and mint, then season with salt and pepper.
Rest the risotto with the lid on for a few minutes before serving.