Heat the oil in a frying pan over a high heat. Add the onion wedges with the sugar, chilli powder or paprika and season to taste. Stir-fry for 2-3 minutes until the onions are tender, but still have a little bite. Drain on kitchen paper.
Preheat the grill to high. For an impressive presentation, stamp out the potato farls into rounds with a 10cm/4in cutter. Toast on a grill rack until golden brown, turning once and then spread with the butter. Arrange each slice of smoked salmon into a loose rosette and place on top of the buttered potato farls. Place each one in the centre of a warmed serving plate and add a dollop of the creme fraiche. Season with pepper and scatter over the onions, then garnish with chives to serve.