Grate the potatoes onto a large flour sack towel or kitchen towel.
Fold up the 4 ends of the towel and then twist it over the sink squeezing out as much of the potato liquid as possible.
Add the drained potatoes to a large bowl.
Next, add in the leftover mashed potatoes, flour, buttermilk, baking soda, salt, and pepper.
Mix until the batter is combined. If It’s too thick add more buttermilk and if it’s too thin, add some flour. It should be the consistency of soupy mashed potatoes.
Add the butter to a 10” cast iron skillet and heat until it’s melted over medium-high heat.
Pour about 1 ½ to 2 cups of the potato batter to the cast iron skillet and spread out using a ladle to flatten it. Turn the heat down to medium.
Cook the boxty for 4 to 5 minutes or until golden brown and flip it over and cook for another 4 to 5 minutes or until golden brown and cooked throughout.
Remove from the pan, garnish with optional sliced green onions, slice into triangles and serve.
Notes
Make-Ahead: These are meant to be eaten when they’re done cooking. You can keep them warm uncovered, placed on a rack on a sheet tray, and in the oven at low temperatures (200°) for up to 30 minutes before serving.How to Store: Cover and keep them in the refrigerator for 4 to 5 days. These will freeze well and be covered for up to 3 months. Thaw them in the fridge for 1 day or until thawed.How to Reheat: Placed the desired number of boxty on a rack over a sheet tray and bake in the oven at 400° for 4 to 6 minutes or until hot and crispy.Raw russet potatoes can be used in this recipe.Feel free to use a mashed potato from either Yukon gold or red potatoes.You can substitute half and half or whole milk for the buttermilk.Feel free to make the boxty into smaller sizes, 2” to 3” in diameter, for quicker cooking and easier-to-flip procedures.After flipping over the boxty, finish it in the oven at 350° for 10 to 12 minutes or until browned and cooked throughout.If you do not have a cast iron skillet, a regular 10" non-stick skillet will work great.