Put roughly half the broad beans in a food processor with the olive oil and pulse them roughly for about 20 seconds to make a loose paste. If the paste is too stiff, add a drop of water and pulse again very quickly.
Put the stock in a pan and bring it to simmering point.
Gently heat 100g/4oz of the butter in a separate pan, add the onions and garlic with a little salt and fry gently until tender. Do not let them brown.
Turn the heat up high and pour in the rice. Stir it with a wooden spoon for about half a minute, coating it with the butter; do not let it stick to the pan.
Add the wine and let it bubble fiercely for about a minute, stirring gently all the time.
Quickly stir in the broad bean paste, if using, then reduce the heat and start to add the hot stock in stages.
When the rice is done, remove from the heat, add the rest of the butter and cover the pan until it has melted.
Stir it in with the broad beans and mint, then add the Parmesan and some seasoning.
Serve immediately.
