First Step: Prepare the Marinade In a medium bowl, whisk together the chopped onion, garlic, olive oil, balsamic vinegar, soy sauce (or coconut aminos for a soy-free option), Dijon mustard, fresh rosemary, salt, and ground black pepper. Combining these ingredients thoroughly ensures the marinade has a balanced tangy, savory, and aromatic profile that will deeply flavor the steak.

Second Step: Marinate the Flank Steak Place the 1 ½ pounds of flank steak into a large resealable plastic bag or a shallow dish. Pour the prepared marinade over the steak, making sure it is fully coated on all sides. Seal the bag or cover the dish, then refrigerate. For optimal flavor and tenderness, marinate for at least 30 minutes, but allowing it to sit up to 2 days will maximize the taste. If you are short on time, aim for a minimum of 4 hours. The acidic balsamic vinegar helps break down the fibers in the steak, enhancing juiciness and tenderness.

Third Step: Preheat and Prepare the Grill Preheat your grill to medium-high heat, about 400°F (200°C). Before placing the steak on the grill, lightly oil the grates to prevent sticking. This step is critical to ensure the steak cooks evenly and releases easily from the grill surface.

Fourth Step: Grill the Flank Steak Remove the steak from the marinade, shaking off excess liquid but reserving the marinade for occasional basting. Place the steak on the grill and cook for approximately 6 to 8 minutes per side. During grilling, brush the steak with the reserved marinade to keep it flavorful and moist. Monitor the internal temperature with a meat thermometer; aim for 150°F (65°C) for medium doneness. Adjust cooking times for your preferred level of doneness. Grilling over medium-high heat produces a flavorful crust while maintaining tender juices inside. For tips on grilling techniques, see How to Grill Flank Steak Perfectly.

Fifth Step: Rest and Slice After grilling, transfer the steak to a cutting board and let it rest for 5 minutes. Resting allows the juices to redistribute evenly throughout the meat, preventing dryness. When ready, slice the steak thinly against the grain. Cutting against the grain shortens muscle fibers and increases tenderness, creating a more enjoyable eating experience.

Final Step: Serve and Enjoy Serve the grilled flank steak with your choice of side dishes like grilled vegetables or a fresh green salad. Leftover steak is excellent for sandwiches or stir-fries. For additional inspiration, visit our shrimp and vegetable stir fry recipe for a complementary dish.