Cut the woody ends off the asparagus. Wash the asparagus and cut into diagonal slices. Peel and wash the prawns. Leave tail fin on as desired. Peel and chop the onion.
Melt 50 g butter in a pot and fry the onion until transparent. Add rice and steam for about 2 minutes at medium heat. Stir in saffron and deglaze with white wine. Boil down wine and add stock in portions, boil down each time.
Season with salt and pepper. Cook the risotto in this way for about 20 minutes while stirring constantly. Melt 40 g butter in a pan and cook the asparagus for about 8 minutes. Season with salt and pepper and add to the risotto.
Fry the prawns in the pan for about 3 minutes. Season with salt and pepper. Place the risotto and prawns on plates and garnish with chervil as desired.