Halve the potatoes, unpeeled and cut them into 1-inch wedges. In a large roasting pan heat 1/3 of the oil in the middle of a preheated 425 oven for 5 minutes. Add potatoes, tossing them to coat them with the oil. Roast them, stirring them every 10 minutes, for 30 minutes or until they are tender. Let the potatoes cool in the pan. In a blender puree the garlic, the vinegar, the rosemary and salt to taste with motor running add the remaining 1/3 cup oil in a stream. Blend the dressing until it is emulsified. In a small bowl of ice and cold water let the onions soak for 5 minutes, drain it well and pat it dry. In a pot of boiling salted water boil the green beans for 5 minutes or until they are crisp-tender, drain them in a colander. Refresh the beans under cold water and pat them dry. In a very large bowl combine the potatoes, the onion, and green beans. Add dressing and toss the salad gently. Serve the salad garnished with the rosemary sprigs at room temperature