Step 1: Prepare the Marinade

• Combine the ingredients: In a small bowl, whisk together the soy sauce, olive oil, Dijon mustard, minced garlic, rosemary, black pepper, and kosher salt.

Step 2: Marinate the Steak

• Add the steak to a bag: Place the flank steak in a large resealable plastic bag or a shallow dish.
• Pour the marinade: Pour the marinade over the steak, ensuring it’s fully coated.
• Refrigerate: Seal the bag or cover the dish and refrigerate for at least 4 hours, preferably overnight, for maximum flavor.

Step 3: Preheat the Grill or Broiler

• Set up your grill: Preheat the grill to medium-high heat. If using the broiler, preheat your oven’s broiler.

Step 4: Cook the Steak

• Grill or broil: Remove the steak from the marinade and discard the excess marinade. Grill or broil the steak for 4–5 minutes per side for medium-rare, or until it reaches your desired doneness. Use a meat thermometer for accuracy (130°F for medium-rare).

Step 5: Rest and Slice

• Let it rest: Transfer the steak to a cutting board and let it rest for 5–10 minutes to allow the juices to redistribute.
• Slice thinly: Slice the steak thinly against the grain for maximum tenderness.

Step 6: Serve

• Plate and serve: Arrange the slices on a platter and serve immediately.