Grate the unpeeled potato onto a clean tea towel. Bring up the edges of the towel, then squeeze over the sink to remove any excess water in the potatoes. Tip into a bowl and add the flour and mustard or horseradish. Season well and mix together.
Divide the mixture into 8 balls and flatten between your hands. Heat a large frying pan with the butter and oil, then add the potatoes to the pan. Cook for 2-3 mins on each side, over a medium heat, until golden.
Stack a couple of hash browns on each serving plate and top with a slice of smoked salmon, a dollop of soured cream or creme fraÃ®che and some chives to serve.