Pulse the stock vegetables together in a food processor until fine. Put with the bay leaf into a 6 litre pot and cover with water, at least 4 litres. Season and bring to the boil. Boil for 30 minutes, then sieve. Discard the vegetables and return the stock to the boil, reducing it to concentrate the flavour until you are left with about 2 litres. Cool and refrigerate if not using immediately.
To make the risotto, heat the stock to a simmer. Cut the asparagus stalks into 5 mm slanted slices. Take your chosen pan and add half the butter and the onion. Sweat over a medium heat for 5 minutes, then add the asparagus. Sweat for 5 minutes more, then add the rice. Sweat, stirring, until the rice sticks, then add 2 ladles of stock. Stirring, cook until the liquid is absorbed and the rice begins to stick. Repeat as for the previous recipes.
When the rice is nearly done turn off the heat and add the remaining butter and then the grated Parmesan. Season, stir, cover and leave for 3 minutes.