Peel the potatoes, onion, and garlic, then roughly chop and add to a food processor. Process until no large chunks remain. Strain and discard most of the liquid through a fine mesh sieve.
Add potato and onion mixture to a large mixing bowl, along with egg, flour, salt, black pepper, and baking soda. Mix until consistent.
Heat ¼ cup oil in a large skillet until shimmering. Spoon approximately ¼ cup of batter into the pan for each pancake. Flatten with the back of a spoon. Fry 2-3 minutes on each side or until golden brown.
Drain deruny on a paper towel-lined plate to remove excess oil, and serve while hot with a dollop of sour cream or applesauce.
Notes
Don't discard all of the liquid - the texture should be pourable like a batter, but not runny.
If you don't have a food processor, you can use the fine side (star side) of a box grater.
If using a box grater, grate onion first, as it will help prevent the potatoes from turning gray due to oxidation.
This is one dish that you should not prep ahead of time. To prevent the potatoes from oxidizing and turning dark, prepare the potato mixture right before you intend to cook them.
When frying the potato pancakes, use medium-high heat. Don’t crowd the pan, or the temp will drop, and as a result, the deruny will become soggy.
You will need to add more oil after every 4-6 pancakes.
To keep deruny crisp, you can set them on a wire rack in a 200°F (95°C) oven while you fry the rest.