Wash and peel the asparagus and cut off the woody ends. Cut the asparagus into pieces. Peel and chop the onion. Heat 2 tablespoons of olive oil. Fry the asparagus in it at medium heat all around for about 8 minutes, the asparagus should still be crisp.
Take it out.
Sauté the onion in the frying fat. Remove. Mix vinegar and grape seed oil. Season with salt, pepper and some sugar. Stir in onion. Mix asparagus and vinaigrette. Leave to stand for about 20 minutes.
Rinse the fillets in cold water and dab dry. Heat 2 tablespoons of olive oil in a frying pan. First fry the fillets on the skin side for 3-4 minutes until crispy, then turn them over and fry them on the meat side for another 4-5 minutes.
Season with salt and pepper.
Clean and wash the frisée salad, spin dry and pluck into small pieces. Fold into the asparagus salad. Cut the fillets into thin slices and arrange on top.