Place the eggs in a small saucepan and cover with cold water. Bring to a 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 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.
Slice each egg in half lengthwise and gently remove the yolks with a teaspoon, keeping the whites intact; set the whites on a serving platter.
Place 4 of the yolks in a bowl with the mayonnaise and Dijon mustard and mash with a fork until smooth.
Finely chop the gherkin and add it to the yolk mixture.
Finely chop the red onion, measure 1–2 teaspoons, and add to the yolk mixture.
Finely chop the smoked salmon and fold it into the yolk mixture until evenly combined.
Spoon the filling into a zip-top bag or piping bag, snip off a corner, and pipe or spoon the mixture into the egg white halves.
Garnish with fresh dill and a grind of black pepper, then serve.
