Cook the potatoes in salted boiling water for 15 to 20 minutes, or until tender. Drain in a colander and cut into 2cm slices. Trim and finely slice the spring onions, pick and finely chop the herbs and roughly shred the trout. Heat the oil and butter in a frying pan over a medium heat and sauté the spring onions for 2 to 3 minutes, until softened. Season generously with sea salt and black pepper. Add the potatoes to the pan and cook for 15 to 20 minutes, turning occasionally, until starting to turn golden and slightly crisp. Make the horseradish dressing: place all the ingredients in a bowl and stir to combine. Add the smoked trout to the pan, stir into the potato mixture, and continue to cook for 5 minutes. Scatter the chopped herbs over the hash before serving with the creamy horseradish dressing. Cut the lemon into wedges for squeezing over.