Zest and juice the unwaxed lemons. Peel and cut the potatoes into 3cm pieces. Trim the green beans.
- 1 Heat the oven to 190°C/375°F/gas 5 (170°C for fan ovens).
- 2 Arrange the potatoes in a single layer in a shallow roasting tin and tuck the rosemary sprigs in between. Drizzle over one-quarter of the oil and season.
- 3 Roast for 30-40 minutes, turning occasionally, until the potatoes are golden and tender.
- 4 Heat one-third of the remaining oil in a non-stick frying pan and cook the trout for 2-3 minutes each side.
- 5 Remove the trout with a slotted spoon and keep warm.
- 6 Boil the beans for 3-4 minutes.
- 7 Divide the potatoes, watercress and beans among serving plates.
- 8 Remove the skin from the trout, break the flesh into chunks and arrange on top of the salad.
- 9 Whisk together the remaining oil, yogurt, lemon rind and juice, and horseradish sauce; season.
- 10 Drizzle the sauce over the salad to serve.
Delicious! I doubled the potatoes and horseradish and added spinach leaves to the watercress. Very tasty.