Cut the herring fillets into small bite-sized cubes, reserving a few pieces for garnish.
Dice the cooked potatoes.
Peel and dice the apples, then toss them with the lemon juice to prevent browning.
Chop three of the hard-boiled eggs into small cubes and reserve one egg for decoration.
Finely chop the onion.
Combine the potatoes, apples, eggs, onion, and herring in a large bowl.
Fold in the tartar sauce until everything is evenly coated.
Arrange the salad on a serving platter.
Garnish with sliced hard-boiled egg, reserved herring pieces, and fresh parsley or dill.
Chill for at least 30 minutes before serving.
