In a large mixing bowl, combine your leftover mashed potatoes, egg, all-purpose flour, and any optional ingredients like grated cheese or chopped green onions. Use a fork or spatula to blend everything together until you achieve a smooth and cohesive mixture.

Place a non-stick skillet over medium heat and add a generous splash of oil or a pat of butter. Allow the skillet to heat for about 2-3 minutes until the oil shimmers or the butter is fully melted and bubbling.

Using a 1/4 cup scoop or measuring cup, take portions of the potato mixture and gently drop them into the hot skillet. Flatten each scoop slightly to form pancake shapes.

Let the pancakes cook undisturbed for about 3-4 minutes, or until the bottoms are golden brown and crispy. Once they reach the desired golden hue, it's time to flip them.

Gently flip each pancake using a spatula and allow them to cook for another 3-4 minutes until they are crispy and golden on this side as well.

Once cooked, transfer your mashed potato pancakes to a plate lined with paper towels to absorb any excess oil. Serve warm, topped with your choice of sour cream, salsa, or any favorite garnishes.