Wash the asparagus, cut off the woody ends and peel the lower third. Cut the asparagus into bite-sized pieces. Cook in boiling salted water for about 10 minutes. In the meantime peel and finely chop the onion.
Fry the onion and lentils in 2 tablespoons of hot oil. Add stock, simmer for about 15 minutes and drain. Wash the fish, dab dry and cut into small pieces. Season with salt and pepper and fry in hot fat on each side for about 2 minutes.
Mix lemon juice, lentils and remaining oil. Arrange asparagus, fish and a few leaves of salad on plates. Pour lentil vinaigrette over the prepared ingredients. Serve garnished with lemon wheels.