Preheat oven to 425F. In a large bowl, combine the potatoes and onions. Toss the potatoes and onions with 1 tablespoon olive oil to coat. Transfer the coated potatoes and onions to a baking sheet. Sprinkle with garlic powder, salt and pepper. Bake in the preheated oven for 10-15 minutes, until the potatoes are slightly tender. Place the green beans in the large bowl from the potatoes and toss to lightly coat, adding more oil if needed. After the potatoes are tender, toss in the string beans to the pan. Bake for another 10-15 minutes, until the beans are tender. Remove from the oven and return the vegetables to the large bowl and slowly add the dressing, tossing and tasting. There is more dressing than I think is needed just for the salad but works out wonderfully if used to marinate the salmon.