Preheat oven to 200ºC. Place potato in a large saucepan with enough water to cover. Bring to the boil. Reduce heat and simmer 5 minutes. Drain and return potato to pan. Drizzle with oil and toss to coat. Lay potato out on a baking sheet. Roast until tender, turning several times, about 25 minutes.
Season inside of trout with tarragon, salt and pepper. Cut four 30 cm squares of aluminium foil. Divide half the fruit slices among foil squares, lay fish on top and cover with remaining slices. Sprinkle orange juice over the top.
Wrap fish in foil and seal packets. Lay packets on second baking sheet. Bake for 20 minutes. Meanwhile, coarsely grate the cucumber into a sieve; press out excess water. Combine cucumber, yogurt and mint in a small bowl.
Arrange the fish and potatoes on serving plates. Spoon on cucumber sauce and garnish with watercress and fruit slices.