Flat Iron Steak Sandwich

I love a flat iron steak for sandwiches, and the horseradish sauce just takes this one to a whole new level! This is the perfect meal for a hungry group. Tip: Cutting across the grain simply means finding the lines in the meat, and cutting parallel to those lines!

Flat Iron Steak Sandwich

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  • Prep Time


  • Total Time


  • Servings



  • ½

    cup mayonnaise

  • 3

    Tbsp prepared horseradish

  • 1

    tsp Worcestershire sauce

  • 1

    tsp Dijon mustard

  • 3

    tsp salt

  • 1-½

    tsp black pepper

  • 4

    cups sliced onions (¼”-thick slices)

  • 2

    Tbsp butter

  • 2

    lbs flat iron steaks, room temperature

  • 2

    Tbsp oil

  • 8

    (5–6” long) club rolls, split vertically ¾ of the way through

  • 2-½

    cups shredded Cheddar cheese


Preheat the oven to 350°F. Combine the mayonnaise, horseradish, Worcestershire, mustard, 1/2 tsp salt, and 1/4 tsp black pepper in a small bowl. Whisk until smooth and refrigerate until ready to serve. Melt the butter in a medium-size skillet over medium heat. Add the onions, 1/2 tsp salt, and 1/4 tsp pepper to the pan. Sauté the onions until they’re browned and caramelized, about 15–20 minutes, stirring occasionally. Season the steaks on both sides with 2 tsp salt and 1 tsp pepper. Heat the oil in a large oven-proof skillet over medium-high heat. Add the steaks to pan and sear for about 4–5 minutes on each side, or until they’re browned. When second side has browned, place the skillet into oven. After 5 minutes, flip the steaks and continue to cook until the internal temperature has reached 135°F (medium-rare) or 140°F (medium), about 10–15 minutes. Remove the pan from the oven, cover the steaks with foil, and let them rest for 5 minutes. While the steaks are resting, place the rolls on a baking sheet, cut side up, and divide the cheese among the rolls. Then divide the caramelized onions among the rolls. Place the baking sheet with the rolls into the oven and bake until the cheese has melted, about 5–8 minutes. When the steaks have finished resting, transfer them to a cutting board and thinly slice the steaks across the grain. Divide the meat among the rolls and top with about 1 Tbsp of the horseradish sauce.


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