Put the trout fillets in a small frying pan and cover with cold water from the tap. Add the spring onions and peppercorns, squeeze in the lime juice and sprinkle in the salt, then bring to the boil, uncovered. When the pan is bubbling, turn the fillets over, then remove the pan from the heat and leave to stand for 7 minutes. Then take the fillets out of the liquid and leave to cool completely, which could take up to 1 hour. Once cool, the trout will be cooked through, with its flesh desirably tender and coral inside.
While the trout's cooling, make a start on the salad. Toast the pumpkin seeds by tossing them in a dry, heavy-based frying pan on the hob. They will start jumping a little, and will darken and get a smokier taste. It doesn’t take long to toast them, so don’t leave the pan and, indeed, keep giving it a quick swirl. Then transfer to a cold plate.
When you’re ready to unite trout with salad, put the watercress into a large shallow bowl, sprinkle with the vinegar, and toss. Now add the trout, removing the skin and tearing the fish into bite-sized pieces or shreddy bits, as you wish.
Halve the avocado and remove the stone, then spoon the flesh out onto the trout and watercress, or cut it into slices if you prefer. Drizzle the oil over the salad, sprinkle with the salt and half of the toasted pumpkin seeds, and toss gently to mix. Scatter the remaining pumpkin seeds on top, and eat.