Cook the potatoes in a large saucepan of boiling salted water until tender. Drain, transfer to a bowl, and mash with a potato masher.
While the potatoes are still hot mix in the crème fraîche, egg yolk, mustard, and salt and pepper. Mix in the scallions, cheese and thyme.
Add 1 tablespoon of the flour to the mixture to help make the consistency suitable for rolling out and sprinkle the remaining flour on your work surface. Roll the potato mixture out to 2 inch thick using a rolling pin and cut it into circles using a 4-inch cookie or biscuit cutter.
Melt the butter in a frying pan and fry the potato cakes on both sides until golden brown.
Recipe adapted from Clodagh’s Kitchen Diaries, Kyle Books (2013)