String the runner beans, slice them into 2cm/¾in pieces and cook in a pan of boiling water until tender. Drain and set aside.
Heat the olive oil in a large lidded saucepan, then add the onion and cook over a low heat for 3-4 minutes, or until softened.
Stir in the rice and cook for a further 2-3 minutes, stirring continuously.
Pour over the stock, stir the mixture and cover with the lid. Cook over a low heat for 15-17 minutes, or until the rice is just tender.
Stir in the mascarpone (if using), parmesan, butter, broad beans and runner beans and season, to taste, with salt and freshly ground black pepper. Sprinkle over the herbs to serve.