Heat the olive oil in a large pan over medium heat.
Add the finely chopped onion and cook until translucent, about 5 minutes.
Stir in the minced garlic and cook for another minute.
Add the Arborio rice to the pan and cook for 2 minutes, stirring frequently to coat the rice with oil.
Pour in the chopped tomatoes and cook for a further 2 minutes.
Begin adding the warm vegetable stock to the rice, one ladleful at a time, stirring constantly and allowing each addition of stock to be absorbed before adding the next. This should take about 20-25 minutes.
Once the rice is cooked and creamy, stir in the mascarpone cheese until well combined.
Season with salt and pepper to taste.
Serve hot, garnished with fresh basil leaves if desired.