Steak Sandwich with Caramelized Onions

This steak sandwich with caramelized onions pairs sliced steak with sautéed mushrooms, melted provolone, arugula, and mustard aioli on crusty ciabatta. Rich, savory, and satisfying.

In this recipe, I’ll show you how to pan-sear the steak, but this sandwich is also a great way to use leftover steak. The rest is easy to customize too, so feel free to swap the cheese or sauce for whatever you have on hand.

Ingredients You’ll Need

For the Steak:

• Steak: 1 to 1 1/2 pounds (454 to 680 grams). For best results, use a tender steak about 1 inch (2.5 cm) thick, like ribeye, filet mignon, New York strip, picanha, Denver steak, or flat iron. You can use cuts like hanger, flank, skirt, or bavette, but only if you slice them very thinly against the grain.
• Salt: Use about 1% of the steak’s weight in salt for dry-brining. For a 1 lb (454 g) steak, that’s about 0.16 oz (4.5 g) of salt, or roughly 1 1/2 teaspoons of Diamond Crystal kosher salt, 1 teaspoon of Morton kosher salt, or 3/4 teaspoon of table salt. I prefer kosher salt because it’s easier to distribute evenly without overdoing it.
• Oil: Use a neutral oil with a high smoke point for searing. I prefer refined avocado oil because it has a neutral flavor and can handle very high heat (around 500°F / 260°C). Check the recipe notes for other good options. Don’t use regular butter for searing because it burns too quickly at high heat.

For the Caramelized Onions:

• Oil: 3 tablespoons (45 ml). I use refined avocado oil, but you don’t need a high-smoke-point oil here. Olive oil works perfectly fine.
• Onions: About 1 1/2 pounds (680 g) of onions, peeled and thinly sliced, preferably yellow onions. Double the amount if you want to make a bigger batch and store the extra in the fridge or freezer.
• Salt: About 1/2 teaspoon (2-3 g), preferably kosher salt. Add it at the beginning to season the onions and help draw out moisture, which helps them soften and caramelize a little faster.

For the Sautéed Mushrooms:

• Mushrooms: 1/2 pound (227 g). I usually use white mushrooms, but any kind will work. Double the batch if you want some extra to store for later.
• Oil: 2 tablespoons (30 ml). I use the same refined avocado oil as for the steak, but olive oil or canola oil works well too.

For the Mustard Aioli:

• Mayonnaise: 1 cup (220-240 g).
• Mustard: 2 tablespoons (30 g). I prefer Dijon.
• Lemon juice: 1 tablespoon (15 ml). Use fresh lemon juice.
• Garlic: 2 peeled cloves, finely minced or grated, or 1/2 teaspoon garlic powder.
• Salt: To taste.

For the Sandwiches:

• Ciabatta rolls: 4-8 small to medium rolls, about 14-18 oz (400-500 g) total. I prefer ciabatta, but you can use any bread you like.
• Cheese: About 4 ounces (113 g) provolone, thinly sliced. Use enough to cover the bread with at least one layer, adding more if needed depending on the size of your rolls. I like provolone here, but American, Swiss, or even blue cheese work well too.
• Arugula: About 2 1/2 cups (50 g) fresh arugula. It adds a fresh, peppery bite to the sandwich. You can use spring mix or other salad greens instead.

Special Equipment You’ll Need

• Wire rack and rimmed baking sheet: For dry-brining the steak.
• 12-inch (30 cm) stainless-steel skillet: For caramelizing the onions.
• Heavy 12-inch (30 cm) skillet: For cooking the mushrooms while the onions caramelize, then searing the steak. A cast-iron skillet is best, but carbon steel or heavy stainless steel are good alternatives.
• Long-handled tongs: For flipping and moving the steak in a hot skillet.
• Meat thermometer: Use an instant-read thermometer for quick temperature checks, or a leave-in probe thermometer to monitor the steak’s internal temperature as it cooks.
• Spatula or wooden spoon

How to Make a Caramelized Onions Steak Sandwich

Prepare the Steak

Pat the steak dry with paper towels, then season both sides with salt equal to about 1% of the steak’s weight. For a 1-pound (454 g) steak, use about 0.16 ounce (4.5 g) of salt, which is roughly:

• 1 1/2 teaspoons of Diamond Crystal kosher salt,
• 1 teaspoon of Morton kosher salt,
• or 3/4 teaspoon of table salt.

Place the steak on a wire rack set over a rimmed baking sheet and refrigerate it uncovered. Dry-brine for at least 1 hour, or preferably 2 to 4 hours for the best results. Since the caramelized onions take about an hour to cook, it’s a good time to let the steak dry-brine while you make the onions and prep the remaining ingredients.

Caramelize the Onions

Trim off the top and root ends of the onions, then cut them in half. Peel them, remove the outer layer, and place each half cut-side down. Slice thinly.

Heat a 12-inch (30 cm) stainless-steel skillet over medium heat for 1-2 minutes. Add the oil and let it heat for another minute. Once the oil is hot and shimmering, add the onions and salt.

Cook for about 3 minutes, stirring occasionally, until the onions begin to soften. Reduce the heat to medium-low and continue cooking, stirring every few minutes, until the onions are very soft and deep golden brown. If they start sticking or look like they might burn, add about 1 tablespoon (15 ml) water, scrape up the browned bits from the bottom of the pan, and keep cooking. Add a little more water as needed to prevent burning.

Exact cooking time depends on your stove and how many onions you’re using, but 1 1/2 pounds (680 g) usually takes about 45-60 minutes. While the onions cook, make the mustard aioli first, then sauté the mushrooms in a separate skillet. This makes the best use of the time while the onions caramelize.

Sauté the Mushrooms

Slice the mushrooms 1/4 to 1/2 inch (0.6-1.3 cm) thick. Heat a skillet over medium-high heat for 1-2 minutes, then add 2-3 tablespoons of oil. Let the oil heat for about 1 minute, then add the mushrooms. Cook for about 15 minutes, stirring every few minutes, until the mushrooms are nicely browned and tender. Once they’re done, transfer them to a bowl and set aside.

Make the Mustard Aioli

Combine 1 cup mayonnaise, 2 tablespoons mustard, 1 tablespoon fresh lemon juice, and 2 garlic cloves, finely minced or grated, in a small bowl. Season with salt to taste, then refrigerate until ready to use.

Cook the Steak

Take the steak and mustard aioli out of the fridge about 15 minutes before cooking.

Turn on the exhaust fan or open a window. Heat a skillet over medium-high heat for 3 minutes. Add 2 tablespoons of oil and heat for another minute. While the skillet heats, pat the steak dry again (the drier the surface, the better the crust).

If the steak has a fat cap, like a New York strip or picanha, hold it fat-side down with tongs and sear for 60 to 90 seconds, until the fat cap is well browned. Then carefully lay the steak flat in the pan and cook, flipping every 30 seconds.

After about 2 minutes, start checking the internal temperature. Insert the thermometer horizontally into the thickest part of the steak, avoiding the bone and large pockets of fat. Once it reaches the pull temperature for your preferred doneness in the chart below, transfer it to a cutting board and let it rest for 5 minutes.

Steak Temperature Chart

Desired doneness	Pull at internal temp	Final temp after resting
Rare	110°F (43°C)	125°F (52°C)
Medium-Rare	120°F (49°C)	135°F (57°C)
Medium	130°F (54°C)	145°F (63°C)
Medium-Well	140°F (60°C)	155°F (68°C)
Well-Done	150°F (66°C)	165°F (74°C)

Assemble the Steak Sandwiches

Once the steak has rested, slice it very thinly against the grain (see note below). Split the ciabatta rolls lengthwise and place both halves cut-side up on a foil-lined baking sheet.

Spread mustard aioli over the bottom halves, then layer on the sliced steak, caramelized onions, sautéed mushrooms, and cheese. Leave the sandwiches open and place them in the oven, as close to the heating element as possible. Broil for 3-5 minutes, just until the cheese melts and the top halves are nicely toasted.

Take the sandwiches out of the oven, spread a little more mustard aioli on the top halves, add the arugula, close them up, and serve right away.

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Try Some of My Other Steak Recipes

• Steak Sandwich with Chimichurri Sauce
• Garlic Butter Steak and Potatoes Skillet Recipe
• Steak and Mashed Potatoes