Leave the asparagus in cold water for a few minutes. Rinse well and snap off the bottom woody section of the spears using your hands. Peel the spears from below the tips.
Cut asparagus into pieces and separate the tops of the spears. Finely chop the onion.
Heat the olive oil in a large pan, then add the onion. When the onion is soft, add the asparagus, and cook for around 10 minutes. Add the white wine and cook till it has mostly evaporated.
Add the rice and toast it for a few minutes until it's almost sticking to the pan. Add a ladleful of hot stock to the rice and the tops of the asparagus, and continue stirring. Once the rice has mostly absorbed the stock, add another ladleful. Keep stirring and adding stock until the rice is cooked. It takes around 18 minutes if you have chosen Vialone Nano.
Season with salt and pepper to taste. Finally, add the Parmesan and butter and stir well. Leave the risotto to rest before serving.
You can ensure this recipe is vegetarian by using vegetable stock. Otherwise, chicken stock can be used.