Place the eggs in a small saucepan and cover with cold water. Bring to a rolling boil over high heat with the lid on.
As soon as the water boils, remove the pan from the heat and keep the lid on. Let the eggs sit in the hot water for 10 minutes.
Drain the hot water and plunge the eggs into iced water or run under cold water until fully chilled, then peel the shells carefully.
Halve the peeled eggs lengthwise and gently remove the yolks with a teaspoon, keeping the whites intact; set the whites on a serving platter.
In a bowl, mash 4 of the egg yolks with the mayonnaise and Dijon mustard until smooth.
Finely chop the gherkin and add it to the yolk mixture.
Finely chop the red onion, measure 1–2 teaspoons, and stir into the yolk mixture.
Finely chop the smoked salmon and fold it into the yolk mixture until evenly combined.
Spoon the mixture into a zip-top bag, snip a corner, and pipe or spoon the filling into the egg white halves.
Garnish each devilled egg with fresh dill and a grind of black pepper, then serve.