In a large bowl, mix together the mashed potatoes, flour, eggs, salt, and black pepper.
Heat oil in a large skillet over medium heat. Once hot, drop in ¼ cup of potato batter and slightly flatten to create a pancake. Cook 4-5 minutes per side, or until golden brown, then repeat with remaining batter.
Make the onion-chive dip by adding all ingredients to a bowl and whisking to combine. Serve mashed potato cakes warm with dipping sauce.