In a medium bowl, wisk together the mashed potatoes and egg. Add the flour and onion and mix all together well. Add salt and pepper to batter if desired.
In a large skillet or on a griddle, heat vegitable oil (about 3 Tablespoons maybe) over medium-high heat. Dollop batter onto the heated surface in heaping tablespoons. Flatten each dollop slightly with the spoon to create the cakes.
Cooking time is about a minute per side. They should be golden brown on each side, so I usually cook them with my eyes more focused on the pancakes than the clock.
If cooking multiple batchs, transfer completed cakes to a paper lined microwave-safe plate in the microwave. If needed when all the cakes are done, microwave the stack for 20 seconds to assure they're all warm.