Separate the eggs.
Beat yolks in a narrow container with a mixer. Gradually add oil in a thin stream. Work on highest speed until a creamy mixture is obtained.
Season with salt, mustard and juice of one lemon. Stir in the yogurt with a whisk.
Chop the anchovies, cornichons, capers, parsley, tarragon and boiled eggs and fold into the mayonnaise.