Steak Sandwich with Caramelized Onions and Horseradish Sauce finished and ready to serve

Steak Sandwich with Caramelized Onions and Horseradish Sauce

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Steak Sandwich piles thinly sliced seared flank steak and slow-caramelized onions onto toasted ciabatta with melted provolone and a tangy horseradish sauce.

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Real caramelized onions take real time — a full 25 to 30 minutes of patient, low-and-slow cooking — and that's the difference between this sandwich and a quick weeknight version. Paired with a punchy horseradish sauce and peppery arugula, it turns simple seared flank steak into something worth sitting down for.

Don't rush the onions; a splash of balsamic at the end brightens all that deep caramelized sweetness. Broil the assembled sandwich just long enough to melt the cheese, then finish with the cold, sharp horseradish sauce and arugula for contrast against all that richness.

Prep20 minutes
Cook40 minutes
Total1 hour
Serves4 servings
DifficultyMedium

Ingredients

  • 1½ pounds flank steak
  • 1½ teaspoons kosher salt
  • ¾ teaspoon black pepper
  • 1 teaspoon garlic powder
  • 1 tablespoon neutral oil
  • 2 large yellow onions, thinly sliced
  • 2 tablespoons unsalted butter
  • 1 teaspoon brown sugar
  • 1 tablespoon balsamic vinegar
  • 4 ciabatta rolls, split
  • 8 slices provolone cheese
  • 2 cups baby arugula
  • ½ cup mayonnaise
  • ¼ cup sour cream
  • 2 tablespoons prepared horseradish, drained
  • 1 teaspoon Dijon mustard
  • 1 teaspoon fresh lemon juice
  • ¼ teaspoon kosher salt
  • ¼ teaspoon black pepper

Instructions

  1. 1Stir together the mayonnaise, sour cream, horseradish, Dijon, lemon juice, salt, and pepper. Refrigerate while the sandwich components are prepared.
  2. 2Melt the butter in a skillet over medium-low heat. Add the onions and brown sugar. Cook for 25–30 minutes, stirring every few minutes, until deep golden.
  3. 3Stir the balsamic vinegar into the onions and cook for 1 minute. Transfer to a bowl.
  4. 4Pat the flank steak dry and season with salt, pepper, and garlic powder.
  5. 5Heat the oil in a cast-iron skillet over high heat. Cook the steak for 4–6 minutes per side, until 128–133°F for medium-rare. Rest for 10 minutes.
  6. 6Toast the ciabatta rolls cut-side down in the skillet for 1–2 minutes.
  7. 7Slice the steak very thinly across the grain. Divide it among the bottom rolls and top with caramelized onions and provolone.
  8. 8Broil the open sandwiches for 1–2 minutes, until the cheese melts. Add arugula, spread the top rolls with horseradish sauce, and close the sandwiches.

Pro Tips

  • Give the onions their full 25–30 minutes over medium-low heat. Rushing with higher heat browns them unevenly instead of building that deep, sweet caramelization.
  • Slice the rested flank steak thin and against the grain — it's a long-fibered cut, and this is what keeps every bite tender.
  • Add the arugula and horseradish sauce after broiling, not before. Both are meant to stay cool and fresh against the hot, melty filling.

Substitutions

flank steak → skirt or sirloin steak Both slice well thin; adjust cook time slightly for thickness.
prepared horseradish → horseradish sauce or extra Dijon mustard Prepared horseradish sauce is milder; Dijon swaps the heat for tang.
provolone → Swiss or white cheddar Both melt well and pair nicely with the horseradish sauce.

Storage Instructions

Store the components separately in the refrigerator for up to 3 days — steak, onions, and sauce each in their own container. Reheat the steak and onions before assembling fresh sandwiches.

Make Ahead

The caramelized onions and horseradish sauce can both be made up to 3 days ahead and refrigerated. Sear the steak fresh for the best texture.

Frequently Asked Questions

Can I make Steak Sandwich with Caramelized Onions and Horseradish Sauce ahead of time?

The caramelized onions and horseradish sauce can both be made up to 3 days ahead and refrigerated. Sear the steak fresh for the best texture.

How do I store leftover Steak Sandwich with Caramelized Onions and Horseradish Sauce?

Store the components separately in the refrigerator for up to 3 days — steak, onions, and sauce each in their own container. Reheat the steak and onions before assembling fresh sandwiches.

Can I freeze Steak Sandwich with Caramelized Onions and Horseradish Sauce?

Caramelized onions freeze well for up to 2 months. Cooked flank steak can be frozen but is best fresh; the horseradish sauce doesn't freeze well.

What should I serve with Steak Sandwich with Caramelized Onions and Horseradish Sauce?

Kettle chips or a simple green salad round out the plate. A crisp lager or a bold red wine both work well alongside.

Can I adjust Steak Sandwich with Caramelized Onions and Horseradish Sauce for the ingredients I actually have?

Mostly, yes. The flank steak is the most forgiving place to improvise, followed by the garlic powder — see the Substitutions section for the swaps that hold up. Keep the ratios close and taste before serving.

What is the most common mistake when making Steak Sandwich with Caramelized Onions and Horseradish Sauce?

Give the onions their full 25–30 minutes over medium-low heat. Rushing with higher heat browns them unevenly instead of building that deep, sweet caramelization.

How long does Steak Sandwich with Caramelized Onions and Horseradish Sauce take?

About 1 hour from start to finish, including 40 minutes of active cooking. Most of it is hands-on, so plan to stay near the stove.

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