Preheat the oven to 270°F (140°C/Gas Mark 1). Peel and grate the potatoes and finely chop the onion; put them both in a sieve. Press with a spoon to squeeze out as much starchy liquid as possible.
Transfer the mixture to a bowl, add the egg, flour, salt, and pepper to taste, and mix well.
Pour the oil into a frying pan to a depth of about 3/8 in. (2 cm) and heat until it shows a haze.
Put a tablespoon of the potato mixture into the oil, flattening it to a small pancake about 2 in. (5 cm) in diameter. Keep adding more tablespoons of the mixture to the pan, cooking four to six at a time. Fry for about 1 minute or until they are golden on the bottom, then turn them over and cook the other side until crisp and golden but still soft in the center.
Remove the pancakes from the pan, drain them on paper towels, and keep them warm in the oven while you cook the remainder.
Cut the salmon into small strips. Rinse and dry the dill or parsley. Serve each pancake topped with a spoonful of sour cream and a few strips of salmon, and garnished with some dill or parsley.