Makes 6 servings
Preheat the oven to 425 degrees. Scrub the potatoes and cut them in quarters or, in eighths (but try to cut them into even sizes). Place them in a single layer in the baking dish.
Scatter the garlic, 3 tablespoons of the olive oil, and the salt and pepper over the potatoes, and toss. Roast for 30-40 minutes, tossing gently every 10 minutes.
Beat the vinegar and mustard in a large bowl. Whisk in the remaining olive oil until smooth. Add the roasted potatoes and oil. Beat the vinegar and mustard in a large bowl. Whisk in the remaining olive oil until smooth. Add the roasted potatoes and mix gently. Season, if desired, with additional salt and pepper, and cool to room temperature. Just before serving, fold in the green onions, chives, and rosemary.