Cut the asparagus in half, at an angle, separating the tips from the thicker stalks. Reserve the tips. Put the stalks in a saucepan with the stock and bring to the boil. Boil for 5 minutes, then reduce the heat to a simmer. Remove the asparagus with a slotted spoon and process in a food processor or blender until puréed.
Melt half the butter in a heavy-based saucepan over a low heat. Add the onion and cook for 10 minutes until softened. Add the rice and cook, stirring, for 1 minute. Add the wine and cook, stirring, until absorbed. Stir in the puréed asparagus.
Add 2 ladlefuls of the simmering stock. Slowly simmer, stirring constantly, until the stock has been absorbed and the rice parts when a wooden spoon is run through it. Add another ladleful of stock and continue to cook, stirring and adding the stock in ladlefuls, reserving 2 ladlefuls, for 16–18 minutes until the rice is creamy and almost tender to the bite.
Add the peas and the reserved asparagus tips and stock and continue cooking until the stock is almost absorbed. Remove from the heat and stir in the Parmesan, mint and remaining butter. Stir vigorously for 15 seconds. Cover with a tight-fitting lid and leave to stand for 2 minutes. Serve immediately with extra Parmesan on the side.