Position racks in the lower third and top of the oven; preheat the broiler. Toss the carrots, parsnips, turnip, olive oil, chopped rosemary, 1/2 teaspoon salt and a generous amount of pepper on a rimmed baking sheet. Roast on the lower oven rack, 10 minutes.
Meanwhile, halve the steak lengthwise and poke a few times on both sides with a fork; transfer to a resealable plastic bag. Add the bourbon, brown sugar, mustard and garlic; tear the rosemary sprig in half and add to the bag. Seal and vigorously massage the ingredients into the meat for 5 minutes to tenderize. Transfer the steak to a foil-lined baking sheet, discarding the marinade. Season with salt and pepper.
Leaving the vegetables in the oven, broil the steak until a thermometer inserted sideways into the center registers 125 degrees F to 130 degrees F for medium rare, 4 to 6 minutes per side. Transfer to a cutting board to rest, 5 minutes. Move the vegetables to the top oven rack and broil, stirring occasionally, until lightly charred, 6 to 8 minutes.
Slice the meat and serve with the vegetables. Top with parsley and serve with mustard.