Boil the eggs for 3 minutes, then drain and cool them under cold running water. Peel the eggs and place in salted water and set aside until required.
Meanwhile, put the asparagus in a pan of boiling water and cook over a medium heat for 3—5 minutes or until tender. Drain, refresh under cold running water and set aside.
Make the dressing by putting the lemon rind and juice, mustard and egg yolk in a food processor or blender. Process briefly to combine the ingredients thoroughly, then, on a low speed, add the oil in a steady stream. Season to taste.
Separate the lettuce and endive leaves, removing any tough stalks, and wash thoroughly. Put the leaves in a bowl and add two-thirds of the dressing. Toss to distribute the dressing evenly, then arrange in the centre of 4 serving plates.
Divide the salmon into 4 and arrange it on the salad leaves. Sprinkle with lime juice. Halve the eggs and arrange them over the salad. Drizzle with the remaining dressing, garnish with chervil sprigs and serve.