Cook potatoes in boiling water until tender, about 10-15 minutes; drain. Let cool slightly. Cut warm potatoes into 1/4-inch thick slices; place in medium bowl. Cook green beans in boiling water until tender, about 5 minutes; drain. Add green beans and onion to potatoes. Pour 1/4 cup dressing over vegetables; toss to coat. Let stand 15 minutes or chill until serving.
Arrange potato mixture, tomatoes, trout or salmon, and eggs on serving platter or individual serving plates; garnish with olives. Drizzle with remaining dressing.
