Grate the onion with the large side of the grater over a large towel or cutting board. Fold on the shredded onion in the towel and drain out the excess moisture then place in to a mixing bowl.
Get a new towel and and grate the peeled potatoes and then do the same with the towel, squeezing out the excess moisture into the sink. Shred all the potatoes, then place into the bowl with the onion.
Add the egg to the mixture and fold in, then season with salt and pepper. Keep mixing.
Heat the frying oil in a frying pan along with the butter. When the oil is hot scoop balls of your mix and place into the oil, a few at a time. Press them down gently with the spatula to flatten them. Cook for 3-5 minutes on each side until golden brown and crispy. Cook all of them and set onto towels to drain off a little before serving.