• Peel and grate the potatoes using a box grater or a food processor. Place the grated potatoes in a clean kitchen towel and squeeze out excess liquid.
• In a large mixing bowl, combine the grated potatoes, egg, all-purpose flour, salt, and pepper. Mix well to form a thick batter. The flour helps bind the mixture together.
• In a large skillet or frying pan, heat cooking oil over medium heat.
• Spoon portions of the potato batter into the skillet, flattening each portion with a spatula to form pancakes. Cook until the edges are golden brown, then flip and cook the other side until crispy.
• Once the Deruny are cooked and crispy on both sides, remove them from the pan and place them on a plate lined with paper towels to absorb any excess oil.

Meanwhile, prepare the mushroom sauce:

• In a separate pan, heat a bit of cooking oil over medium heat. Add chopped onions to the pan. Cook until soft, then add mushrooms. Sauté until the mushrooms are golden.
• Add sour cream, and season with salt, pepper, and thyme. Stir in minced garlic and cook until the sauce is well combined and heated through.
• Serve the Deruny hot with a dollop of sour cream on top or mushroom sauce.