Peel the potatoes and grate them on a fine grater.
Finely chop the onion and add it to the potatoes. Mix them well. The onion juice will keep the potatoes from going brown.
Add the flour, egg, and sour cream and mix well.
Then add 1 tsp of salt and a pinch of black pepper. The batter should be liquid enough to be ladled easily.
Pour some sunflower oil into a frying pan and heat well.
Using a spoon, form small, flat rounds of potato mixture and fry them like regular pancakes. When the pancakes are brownish on one side, turn them and repeat the procedure.
Serve deruny while hot with a dollop of sour cream.