Step 1 Place the yolks in a small bowl. Use a fork to mash them. Add the sour cream and mayonnaise. Mix until smooth. Stir in the fennel, smoked salmon, onion and capers. Taste; add salt and pepper to taste. (Be careful, as smoked salmon can be very salty).
Step 2 Spoon the mixture into the whites, dividing evenly, and smoothing the surface. (Alternatively, a pastry bag fitted with a fluted tube can be used.) Garnish the eggs with chives.