Heat the duck fat in a saucepan over a medium heat until it melts and its temperature reaches 60C/140F (use a cooks' thermometer to check this). Add the smoked salmon pieces and poach for 4-5 minutes. Remove from the pan with a slotted spoon and set aside to drain on kitchen paper.
For the salad, whisk the caster sugar, rice wine vinegar and water together in a bowl until well combined. Season, to taste, with salt and freshly ground black pepper.
To serve, mix the salad dressing, cucumber and mooli together in a bowl until well combined. Spoon some salad into the centre of each of 4 serving plates. Top each with a piece of poached smoked salmon and scatter over the red amaranth and coriander cress.