Combine 1 1/2 tablespoons olive oil, 1/2 teaspoon salt, 1/4 teaspoon pepper, garlic, and potatoes in a roasting pan or jelly-roll pan; toss well to coat. Bake at 400° for 1 hour and 10 minutes or until tender, stirring after 35 minutes. Cool 10 minutes.
Squeeze garlic cloves to extract pulp. Discard skins. Combine garlic pulp, remaining 1 1/2 tablespoons oil, remaining 3/4 teaspoon salt, remaining 1/4 teaspoon pepper, chives, vinegar, and mustard in a large bowl; stir well with a whisk. Add potatoes to bowl; toss well to coat.