Potato salad with green beans and red onion

Adapted from Epicurious.

3 pounds red boiling potatoes
1 tbs + 1/3 c. olive oil
1 red onion, halved lengthwise and sliced thin lengthwise
1 garlic clove
1/4 c. red-wine vinegar
1 tbs fresh rosemary leaves or 1 teaspoon crumbled dried, plus rosemary sprigs for garnish
1 tbs Dijon mustard
2 lbs green beans, trimmed and cut into 1-inch pieces
24 Kalamata or Niçoise olives, pitted and halved

Halve the potatoes, unpeeled, and cut them into 1-inch wedges.
In a large roasting pan heat 1 tbs of the oil in the middle of a preheated 425°F oven for 5 minutes, add the potatoes, tossing them to coat them with the oil.
Roast for 30 minutes, or until they are tender, stirring every 10 minutes.
Add the sliced onions and turn to coat then let the potatoes cool in the pan.
In a blender purée the garlic, the vinegar, the rosemary leaves, Dijon mustard, and salt to taste, with the motor running add the remaining 1/3 cup oil in a stream, and blend the dressing until it is emulsified.
In a kettle of boiling salted water boil the green beans for 5 minutes, or until they are crisp-tender, and 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, the green beans, and the olives, add the dressing, and toss the salad gently.
Serve the salad, garnished with the rosemary sprigs, at room temperature.

Toddler review-
The toddler devoured this two days in a row now.  It makes a bunch so make sure to share with friends.