Grilled Balsamic Flank Steak (with Onions)

Nothing beats the aroma of grilled meat during the summer months. If you’re looking for an easy, delicious steak recipe, then look no further than my Grilled Balsamic Flank Steak with Onions!

The secret to this dish is that the onions are marinated alongside the flank steak. Cook the onions in a cast-iron skillet until they are brown and soft, then serve them right on top! For the best results, let the steak and onions marinate for at least 30 minutes before grilling.

Why this recipe works

• This Grilled Balsamic Flank Steak recipe is quick and easy to make with a fast marinating time.
• It requires simple ingredients, such as balsamic vinegar, salt, rosemary, and Dijon mustard, etc.
• The marinade is customizable! It can be made ahead and stored, and you can add extra ingredients to suit your tastes.
• You can use other flat steaks, such as hanger steaks, flat-iron, and more.
• The steak and onions are fantastic in sandwiches and wraps or sliced and served over freshly made French fries!
• This recipe can be made on a traditional grill or a Blackstone Griddle.

Ingredients

• 1 1/2 pounds beef flank steak
• 3 medium yellow onions, peeled and cut into 1/2″ slices
• Balsamic vinegar
• Dijon mustard
• Soy sauce
• Salt (fine sea salt or table salt)
• Fresh rosemary, finely chopped (sub with 3/4 teaspoon dried rosemary or thyme)
• Garlic powder
• Black pepper
• Olive oil (sub with avocado oil)

Making Grilled Balsamic Flank Steak

• Make the marinade: Combine balsamic vinegar, Dijon mustard, soy sauce, garlic powder, salt, black pepper, rosemary, and olive oil in a small bowl.
• Marinate the steak and onions: Place the thawed steak and onions in a large resealable plastic bag or shallow glass dish. Pour the balsamic marinade over the top and toss to coat evenly. Place it in the refrigerator for 30-40 minutes.
• Preheat your grill or Blackstone griddle to medium-high heat, or 400°F. If you are using a standard grill, place a cast-iron skillet on the grill grate and let it heat up. Add a little oil. It’s ready when you see a wisp of smoke. If you are using a Blackstone Griddle, skip this step.
• Grill the steak: Remove the steak and onions from the marinade. Place the steak on the grill, and brush the reserved marinade over the top. Grill for 8-10 minutes per side, depending on the size and desired doneness. I recommend medium-rare to medium doneness or an internal temperature of 130 to 140°F.
• Cook the onions in a cast-iron pan or on your Blackstone Griddle until golden brown, soft, and slightly caramelized.
• Rest the steak: Once the flank steak and onions are cooked to your preference, remove them from the grill and let the steak rest for 5 minutes. Cover the onions and keep warm.
• Slicing and serving: Slice the steak across the grain and serve it topped with the caramelized onions.

What to serve with Balsamic Flank Steak

One of my favorite things about this dish is that it can be served alongside a variety of side dishes! Here are some of my favorites.

More steak recipes!