Take an empty jam jar with a lid, put in a pinch of salt,
a good grind of pepper, plus the mustard, olive oil and vinegar. Shake and place the jar in the fridge - use as you want, it will last a few weeks.
Cut the beetroot into quarters and place in a bowl. Add in the creamed horseradish and 2 tbsp dressing, stir well. Remove the skin from the trout and break the fish into pieces about the size of a 50p. Mix together the watercress and the beetroot, then top with the smoked trout.