Place the potato pieces in a medium saucepan and cover with salted water. Bring to the boil, then reduce the heat and simmer for about 20 minutes or until tender. Drain and allow the potatoes to steam off for a few minutes in the colander, then tip them back into the pan and mash them with half the butter.
Meanwhile, heat a frypan over a medium heat. Add the remaining butter, followed by the sliced onion and a little salt and pepper. Cook for 6-8 minutes until beginning to soften. Add the sage leaves, stir well and remove from the heat to cool.
Add the flour, blue cheese, baking powder and softened onion to the potato, season well with salt and pepper and mix to combine.
Shape the potato dough into a round that measures about 6" in diameter, then cut into 4 quarters. Wipe out the onion pan and return it to a medium heat with a drizzle of extra olive oil.
Lay the quarters in the pan and cook for 4-5 minutes on each side or until golden brown. The farls are delicious topped with fried eggs and roasted tomatoes.