Put the potatoes in a large pan of salted water. Bring to the boil then turn the heat down and simmer for 15-20 minutes until cooked through.
Drain and place back in the pan. Pass the potatoes through a potato ricer or mash. Beat in the butter and crème fraiche. Season with salt and ground black pepper.
Stir in the spring onions and beat in the flour and baking powder. Tip the potato mix onto a clean work surface and knead gently until the dough is smooth and feeling a little bit more elastic. Divide into two even-sized pieces.
Roll out the first piece of dough on a lightly floured surface to a circle about 18cm diameter. Cut into quarters. Repeat with the remaining dough.
Heat a large non-stick frying pan and dry fry the farls, a few at a time, for about 3 minutes. Turn over and continue to fry for a further 3 minutes until lightly browned and slightly puffy. Serve as soon as possible. You can warm through in a low oven for 5-10 minutes if you can’t eat the farls straight away.