Preheat oven to 220ºC (425ºF). Place the potatoes in a saucepan of cold salted water and bring to the boil. Cook for 12–15 minutes or until just tender. Drain and cool slightly. Place 2 tablespoons of the oil on a baking tray and heat in the oven for 5 minutes. Place beetroot and remaining oil in a bowl and toss to coat. Squash the potatoes with the back of a spoon and carefully add to the hot tray with the beetroot. Roast the potatoes and beetroot for 20 minutes or until the potatoes are crisp and beetroot is tender.
While the vegetables are roasting, make the horseradish dressing. Place the mayonnaise, horseradish, garlic, vinegar, salt and pepper in a bowl and mix to combine. Set aside.
Place the roasted vegetables in a bowl with the trout, broad beans and watercress and toss to combine. Spoon over the dressing to serve. Serves 4.