In a large mixing bowl, combine your cold leftover mashed potatoes with the large egg, stirring well until the mixture is fully blended and smooth.

Gently fold in the grated yellow onion, all-purpose flour, kosher salt, and black pepper into the potato mixture.

In a large frying pan, heat about ½ inch deep of canola oil over medium heat, aiming for around 350°F.

Using a scoop, carefully drop scant ½-cup portions of the potato mixture into the hot oil, flattening them into round patties.

Fry the patties for about 2½ to 3 minutes on one side or until golden brown, then flip and cook for an additional 2 to 2½ minutes.

Once cooked, transfer the patties to a cooling rack over paper towels to drain excess oil.