1. Melt the clarified butter in a frying pan and fry the fishcakes for approximately 8 minutes. In the meantime, transfer the egg yolks, mustard and grated lemon rind to a bowl and mix together.
2. Add the oil drop by drop, whisking fiercely the whole time until the mixture begins to thicken. Finally, season with salt and pepper.
3. Wash, dry and finely chop the tarragon and add to the mayonnaise.
4. Arrange the fishcakes on plates. Serve with the mayonnaise. An ideal accompaniment are fried potatoes.