Flat Iron Steak Sandwich Recipe

Bring steak to room temperature

Remove the 8 oz flat iron steak from the refrigerator 20–30 minutes before cooking so it comes closer to room temperature; pat dry with paper towels.

Season both sides of the steak evenly with 1 tsp salt and 1/2 tsp freshly ground black pepper.

Heat a heavy skillet or cast-iron pan over high heat until smoking hot (about 3–5 minutes). Add 1 tbsp olive oil and swirl to coat.

Place the steak in the pan and sear undisturbed for 3–4 minutes until a deep brown crust forms. Flip and sear the other side 3–4 minutes for medium-rare (internal temp ~130–135°F / 54–57°C). Adjust time if your steak is thicker or to desired doneness.

Remove the steak to a cutting board, tent loosely with foil, and let rest 5–7 minutes. Brush or drizzle with 1/2 tsp olive oil before slicing to keep moist.

Slice the steak thinly against the grain into pieces suitable for sandwich filling (about 1/8–1/4 inch thick).

While the steak rests (or earlier), halve and thinly slice 1 medium yellow onion into half-moons.

Heat 1 tbsp unsalted butter and 1 tsp olive oil in a medium skillet over medium-low heat. Add the sliced onions and 1/4 tsp salt; stir to coat.

Cook slowly, stirring every few minutes, for 20–25 minutes until onions are deeply golden and soft. If the pan becomes dry or onions stick, add a splash of water. Stir in 1/2 tsp brown sugar in the last few minutes if using to accelerate browning and deepen flavor.

When caramelized, remove from heat and keep warm until assembly.

In a small bowl combine 1/4 cup mayonnaise, 1 tsp Dijon mustard, 1/2 tsp hot sauce (or smoked paprika), 1/2 tsp lemon juice, 1/4 tsp garlic powder, and 1/8 tsp salt. Whisk until smooth and taste-adjust for heat, acidity, or salt.

Slice the sandwich roll or baguette lengthwise. If you like, spread 1 tsp butter on cut sides and toast lightly in a skillet over medium heat until golden (about 1–2 minutes).

Spread a generous layer (about 1–2 tbsp) of the spiced mayo on both cut sides of the bread.

Arrange a layer of baby arugula (about 1/2 cup) on the bottom half of the roll.

Top the arugula with the sliced flat iron steak, distributing evenly. Pile on the warm caramelized onions over the steak.

Close the sandwich, press gently, slice in half if desired, and serve immediately.