1.Boil, steam or microwave potatoes and beans, separately, until tender; drain. Rinse under cold water; drain. Halve potatoes lengthways.
2.Meanwhile, for make salsa verde, combine bread and vinegar in small bowl; stand about 5 minutes or until vinegar is almost absorbed. Blend or process herbs, anchovy, garlic, capers and bread mixture to a coarse paste. Stir in oil. Season to taste.
3.Combine potatoes, beans and two-thirds of the dressing in large bowl. Place watercress and lettuce on serving platter. Top with potato mixture and trout; drizzle with remaining dressing.