Grate onion and place in a medium sized bowl.
Grate coarsely potatoes. Alternatively, you can use the food processor to shred potatoes (use the shredding disc).
Combine shredded potatoes with the onion and mix well. (Do it right away so the potatoes don't get dark).
Let the mixture stay for 5 minutes, until the potatoes release juice. The juice will contain the starch too. Separate the potatoes from the juice.
Place the separated shredded potatoes in another bowl.
Check if the juice contains enough starch. If yes, discharge the juice and add the starch to the potato mixture. In this case, you don’t need to add eggs or flour. If there is not enough starch, add the available starch to the mixture, an egg, and a tablespoon of flour. Mix well.
Add salt and black pepper, and mix well again.
Preheat the frying pan, reduce the heat to medium and add vegetable oil.
With a tablespoon, drop the potato mixture in small portions to the skillet. (You can also form pancakes with your hands and place them on the frying pan).
Fry deruny on medium heat on one side for a minute, with a spatula flip them to the other side and fry for 30-40 seconds.
When the deruny is ready, transfer them to a flat plate covered with paper towels (to absorb excess oil).
Brush the deruny with butter.
Serve warm with cold sour cream. Sprinkle with chopped fresh parsley or dill.