Rub both sides of the steak with 1 tablespoon olive oil, 1 tablespoon kosher salt, 1 teaspoon freshly ground black pepper, and 1 teaspoon garlic powder. Let rest at room temperature for 20 minutes (or refrigerate up to 2 hours covered).

Cook

Heat a heavy skillet or cast-iron pan over high heat until very hot (2–4 minutes). Add 1 tablespoon unsalted butter and swirl. Place the steak in the pan and sear without moving 2–3 minutes until a deep brown crust forms. Flip and sear the other side 2–3 minutes for medium-rare (internal 125–130°F / 51–54°C). For medium, cook an extra 1–2 minutes per side. Adjust time for thickness. If steak is thicker than 1 inch, finish in a 400°F / 200°C oven for 3–5 minutes after searing.

Rest and slice

Transfer steak to a cutting board and tent loosely with foil. Rest 8–10 minutes. Slice thinly across the grain into 1/8–1/4-inch strips. Reserve any juices on the cutting board to add to the sandwich sauce if desired.

Marinade / Sauce

While the steak rests, whisk together 2 tablespoons soy sauce, 1 tablespoon red wine vinegar, 1 teaspoon Worcestershire sauce, 1 teaspoon fresh lemon juice, 1 teaspoon minced garlic, 1 tablespoon olive oil, 1 teaspoon red pepper flakes, and 2 tablespoons chopped parsley in a small bowl. Taste and adjust salt. If desired, stir in up to 1 tablespoon of the reserved steak juices for extra beefy flavor. This sauce will be used to dress the steak and as a drizzle for the sandwiches.

Sandwich Build

Prepare rolls

Slice the ciabatta rolls in half horizontally. Lightly toast cut sides in a dry skillet over medium heat or in a 375°F / 190°C oven for 4–6 minutes until golden and slightly crisp.

Make spread

Mix 1/2 cup mayonnaise with 1 tablespoon Dijon mustard in a small bowl. Season the spread with 1/4 teaspoon salt and 1/4 teaspoon freshly ground black pepper. Optionally stir in 1 teaspoon of the steak sauce for extra flavor.

Assemble sandwiches

Spread mayonnaise-mustard mixture evenly on both cut sides of the toasted rolls. On the bottom half layer: a handful (about 1/2 cup) of arugula, 2–3 slices of tomato (seasoned lightly with a pinch of salt and pepper), a few slices of red onion, and optional pickled jalapeños to taste. Pile sliced entraña steak over the vegetables, distributing evenly among the 4 rolls (about 4 ounces cooked steak per sandwich). Drizzle 1–2 teaspoons of the prepared sauce over the steak on each sandwich.

Finish and serve

Close sandwiches and press gently. If desired, place assembled sandwiches back in a hot skillet and press with a spatula for 30–45 seconds per side to warm through and slightly compress. Slice each sandwich in half and serve immediately while steak is warm.