Flank Steak with Roasted Root Vegetables

Name: Flank Steak with Roasted Root Vegetables
Ingredients: 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