The key to crispy deruny is to remove excess liquid from the grated potatoes. The mixture should be thick and not runny so that it holds its shape during frying.
Deruny are traditionally fried in vegetable oil (sunflower oil) and served hot, preferably with a generous dollop of sour cream or yogurt.
Peel the potatoes and onion and finely grate (or blend) them into a pulp.
Transfer the potato-onion mixture to a sieve and let it drain, or squeeze it out through a thin cloth to remove as much liquid as possible. Discard the liquid (keep only the starch settled at the bottom of the bowl).
Transfer the squeezed mixture to a bowl. Add the egg, flour (or the settled starch from the liquid), salt, and pepper. Mix well.
The mixture should be thick, not watery. If it is too runny, add a little more flour.
Heat a sufficient amount of oil in a large pan. The temperature should be medium-high.
Use a spoon to scoop the mixture and form small round pancakes (Deruny). Carefully place them into the hot oil.
Fry for 3-4 minutes on each side until they are golden brown and crispy. The pancakes should be cooked through.
Remove the finished Deruny and place them on paper towels to drain excess oil.
Serve immediately, hot, with a generous spoon of sour cream and sprinkled with chopped herbs.