Put the eggs in a pan and cover with cold water. Bring to the boil, cover with a lid, turn off the heat and let it rest for 8 minutes. Rinse with cold water and drain. (This can be done a day before, but do chill the cooked eggs).
Remove the egg shells, then cut the eggs in half (vertically).
Scoop out the egg yolk into a bowl. Add the yogurt, herb, mustard, cayenne and some ground pepper. Squeeze some lemon juice and taste - adjust it to your liking.
Finely chop the spring onions and add to the egg mixture.
Rinse the crayfish tails, make sure they are shell-less and add to the egg mixture.
Combine gently and taste it again - adjust the seasoning and the cayenne/mustard/yoghurt/lemon if needed.
Fill the egg whites with the mixture, then put them on a serving plate. Scatter some fresh chives/dill on top.
Serve chilled or at room temperature.