1. Preheat oven to 450°. Coat a baking sheet with cooking spray.
2. Toss potatoes with oil; add rosemary and 1/2 teaspoon salt. Place in a single layer on baking sheet. Bake 20–22 minutes or until golden, tossing halfway through. Remove from oven, and gently loosen with a spatula; keep warm.
3. Place oven rack at highest position; preheat broiler. Coat a broiler pan with cooking spray.
4. Whisk together the vinegar, mustard, pepper, thyme, and remaining 1/4 teaspoon salt in a large dish. Add steak; marinate at room temperature 20 minutes, turning occasionally. Broil for 6–7 minutes per side or to desired degree of doneness. Remove from oven; let stand for 5 minutes. Cut into 4 equal portions.
5. Divide steak and fries evenly among plates. Garnish with parsley, if desired. Serve immediately with mustard.