Flank Steak with Roasted Root Vegetables

6 carrots, halved crosswise and lengthwise

4 parsnips, halved crosswise and lengthwise

1 turnip, peeled and cut into 1/2- to 3/4-inch wedges

2 tablespoons extra-virgin olive oil

1 1/2 teaspoons chopped fresh rosemary, plus 1 sprig

Kosher salt and freshly ground pepper

1 flank steak (about 1 1/4 pounds)

2 tablespoons bourbon or low-sodium beef broth

2 tablespoons packed light brown sugar

1 teaspoon horseradish mustard or spicy mustard, plus more for serving

1 clove garlic, smashed

Chopped fresh parsley, for topping