Wash the asparagus, cut off the woody ends. Cut the asparagus into pieces. Cook in boiling salted water for 8-10 minutes. Peel onion and garlic. Cut garlic into thin slices.
Dice onion very finely. Wash the prawns, dab dry. Heat 1 tablespoon of oil with the garlic in a frying pan, fry the prawns on each side over medium heat for about 2 minutes. At the end of the frying time, add the onion.
Season with salt and pepper. Clean, wash and cut the peppers into strips. Whisk vinegar, salt, pepper and sugar. Stir in 5 tablespoons of oil in a thin stream. Mix all salad ingredients, season again.
Divide into glasses and season to taste with salt and pepper. Garnish with chervil.