Mince 1 sprig rosemary to total 2 teaspoons. Cut 1/2 red onion into 1/2 in. thick slices. In large bowl, combine potatoes, carrots, red onion, minced rosemary, garlic, 1 teaspoon salt, 1/2 teaspoon pepper and 2 tablespoons oil; toss to coat. Place vegetables in medium roasting pan. Squeeze 1 lemon half over vegetables.