Boil the potatoes in salted water until just cooked. Drain well and, when cool enough to handle, grate on the coarse side of a cheese grater into a mixing bowl. Add the spring onions, seasoning, flour and half the butter and mix well. Divide into 4 equal balls and pat into flat rounds. Leave covered in the fridge overnight if you can.
Heat a non-stick frying pan over a moderate heat and add the rest of the butter. Gently fry the potato cakes, 2 at a time, for 2 to 3 minutes each side until light brown and heated through. Keep warm.
Meanwhile, soft boil the eggs for 4 minutes – this recipe works best with runny yolks. Arrange a couple of potato cakes on each plate and top with smoked salmon. Shell the eggs and place on top. Spoon soured cream over the eggs and sprinkle over the chives. Season well.