Grilled Flank Steak Sandwiches

These Grilled Flank Steak Sandwiches piled high with caramelized onions and mushrooms are just my way of saying “I love you.”

Grilled Flank Steak Sandwiches with Caramelized Onions and Mushrooms

Have you ever heard of the five love languages? It was a really popular book back in the 90s, or at least it was in the church circles I was a part of.

The book is designed to explain the five primary ways people give and receive love–physical touch, acts of service, gifts, words of affirmation, and quality time.

The idea is once you discover your love language and the love language of those around you, you’re better able to communicate your love to them. The concept makes a lot of sense to me, but I think they should add a sixth love language for FOOD.

One of the primary ways I like to give and receive love is through food. If a friend has had a death in the family, my first response is to bring Cottage Cheese Lasagna. If they’re getting married, I’ll throw them a shower so I can make Easy Strawberry Cake.

New baby? I’ll shower you with meals until your freezer cries out for mercy!

One of my favorite ways to show the Mr. I love him is by bringing him his favorite foods. I love surprising him with Milano cookies (his favorite!), and I can’t resist picking up a specialty beef jerky for him when I come across them.

And more than buying his favorite foods, I love making them too. And this Grilled Flank Steak Sandwich is way up there on his favorites list.

These Grilled Flank Steak Sandwiches are what tailgating dreams are made of.

Drool Worthy Grilled Flank Steak Sandwiches

I don’t know too many guys who wouldn’t flip over this combination of juicy marinated steak, sweet onions, and mushrooms. And truthfully, I don’t know too many girls who wouldn’t abandon all of Emily Post’s rules of etiquette and dive into this head first.

Beef is often my go-to when I want to prepare something quick and filling that I know the Mr. and I will both enjoy, but this was my first time working with flank steak. I’ve always been intimidated to grill steak, but I can tell you after this experience, I’ll be doing it a lot more often.

How To Grill Flank Steak

The trick to a great flank steak is to marinate it first, then grill over high heat until seared on the outside but still nice and rosy in the center.

The marinade I chose takes about five minutes to throw together and results in a juicy, ultra flavorful cut of meat.

 The other trick? Always, always, always let the steak rest for at least 10 minutes before slicing. This applies to any steak, and is the key to keeping moisture in the steak rather than all over  your cutting board.

Finally, a flank steak should be sliced against the grain, which cuts through any tough tendons and gives you the most tender slices of steak.

Now that I know how easy this flank steak is to prepare, it’s going to be making a lot more appearances on my menu plans.

Tips for grilling Flank Steak + A Killer Flank Steak Sandwich

The rest of the meal is also prepared on the grill and is mostly hands off. Since the steak only takes about 10 minutes, I like to start with the caramelized onions.

Did you know you can cook just about anything on the grill if you have a cast iron skillet? I just put my cast iron skillet right on the grates and let the onion cook down for about 15 minutes before adding the mushrooms. In 20 minutes they’ll be soft, sweet, and ready to be piled on your sandwich.

To finish this sandwich off, I like to add some white cheddar cheese (Swiss would also be good) and give it a few minutes under the broiler just to get nice and melty.

The last surprise in this flavor extravaganza is a generous shmear of horseradish mayonnaise. I simply stirred in a little creamed horseradish to my plain mayo, but feel free to use your favorite spread instead.

This Grilled Flank Steak Sandwich was a huge hit in our house!

Looking for more meaty main dish recipes to make on the grill? Try my Pimento Cheese Burgers, Grilled Steak Kabobs with Basil Mint Chimichurri, Marinated Skirt Steak, and Grilled Flank Steak Tacos.

Grilled Flank Steak Sandwiches with Caramelized Onions and Mushrooms

Grilled Flank Steak Sandwiches with Caramelized Onions and Mushrooms

Yield: 5
Prep Time: 15 minutes
Cook Time: 25 minutes
Total Time: 40 minutes


For the steak and marinade:

  • 1.5 lbs. flank steak
  • 1/4 cup olive oil
  • 3 cloves garlic
  • 2 Tablespoons red wine vinegar
  • 1/3 cup soy sauce
  • 2 Tablespoons honey
  • 1/2 teaspoon ground pepper

For the mushrooms and onions:

  • 1 Tablespoons butter
  • 2 Tablespoons olive oil, divided
  • 1 large yellow onion, thinly sliced
  • 8 ounces sliced mushrooms
  • Pinch salt

For the sandwiches:

  • 1 large french baguette, cut into fourths (or two smaller ones cut in half)
  • 8 slices white cheddar cheese
  • 1/4 cup mayo
  • 1/4 sour cream
  • 2 Tablespoons creamed horseradish spread
  • Sliced Tomato


For the steak:

  1. Place the steak in a large ziplock bag or baking dish along with all the marinade ingredient. Toss well to coat. Refrigerate for two hours or overnight.

To grill:

  1. Remove the flank steak from the fridge and allow it to come to room temperature. In the meantime, place a cast iron skillet on the grill, and preheat it to medium high heat.
  2. Add the butter and 1 Tablespoon olive oil to the skillet. Add the onion and pinch of salt and toss to coat. Put the lid on the grill and let the mixture cook for 15 minutes, stirring about every five minutes. If the onions start to brown before they soften, add a Tablespoon of water to the skillet.
  3. Add the mushrooms and an additional Tablespoon of olive oil to the skillet and cook an additional 5-10 minutes or until onions are soft and sweet and mushrooms are golden brown. Remove the skillet from the grill.
  4. Meanwhile, place the flank steak on the grill over the highest heat source and grill for 4-5 minutes per side for medium steak. Remove to a plate and allow to sit at least 10 minutes before slicing.

For the sandwiches:

  1. Preheat the broiler to low heat. Slice the baguettes in half, smear with butter, and toast them if desired. Layer each sandwich with the steak, the mushrooms and onions, and then the sliced cheese. Place them under the broiler for 3-4 minutes or until cheese is melted. Meanwhile, in a small bowl whisk together the mayo, sour cream, and horseradish.
  2. When the sandwiches are done broiling, spread the top half with mayo mixture and add sliced tomatoes. Sandwich and serve.

Did you make this recipe?

Leave a review, or head over to Instagram and share a photo! Tag your projects with #neighborfoodies!

 Flank Marinade Adapted from Simply Recipes

Sunday Supper MovementMonday – Slow Cooker Asian Beef by Family Foodie
Tuesday – Grilled Flank Steak Sandwiches with Caramelized Onions and Mushrooms by NeighborFood
Wednesday – Steak Caprese Salad by Cindy’s Recipes and Writings
Thursday –  Smothered Hamburger Steak by In The Kitchen With KP
Friday – Flank Steak with Chimichurri Sauce by Peanut Butter and Peppers

This post is sponsored by The Beef Checkoff. All opinions are my own.


  1. Iโ€™m confused,
    You say; add mayo, add sour cream, add creamy horseradish sauce.
    My ?
    creamy horseradish sauce Already contains sour cream and mayo.
    So you say in addition to the readymade creamy horseradish sauce, add to it sour cream and mayo.
    It looks delishish.

    1. Sounds like you have a pre-made horseradish spread or sauce rather than just a creamed horseradish product (where the only ingredient is horseradish). I would just use your store bought sauce as is, without mixing in the other ingredients. Should be delicious!

  2. I’m hear you, Courtney on spreading love through food! These sandwiches look “lovely” to me! Oh and I’m with Will on the Milanos, too!

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