Prepare the Steak:

1. Pat the steak dry and 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.
2. Place the steak on a wire rack set over a rimmed baking sheet and refrigerate it uncovered. Dry-brine it for 2 to 4 hours for the best results. One hour is the minimum.
3. Optional: If you don’t have at least 1 hour to dry-brine the steak, skip salting it in advance. Salt the steak immediately before searing, or season it with salt after cooking.

Make the Chimichurri Sauce

1. If you’re dry-brining the steak, make the chimichurri while it’s in the refrigerator. If not, make the chimichurri first and let it rest for 1 hour before serving.
2. By hand: Finely chop the parsley and oregano leaves. Finely chop, grate, or press the garlic. Add them to a bowl with the olive oil, red wine vinegar, red pepper flakes, salt, and black pepper. Mix well, then taste and adjust the seasoning as needed.
3. In a food processor: Add the garlic, parsley, oregano, red pepper flakes, salt, and black pepper. Pulse until finely chopped. Add the olive oil and red wine vinegar, then pulse just until combined. Don’t blend until smooth; keep some texture in the chimichurri.
4. Cover and refrigerate the chimichurri for at least 1 hour before serving.

Cook the Steak

1. Using leftover steak? Skip the cooking step, slice the steak thinly against the grain, and move on to assembling the sandwiches.
2. Take the steak and chimichurri out of the refrigerator about 15 minutes before cooking and let them sit at room temperature.
3. Turn on the exhaust fan or open a window before you start. Heat a heavy skillet over medium-high heat for 3 minutes. Add 2 tablespoons of oil and heat for 1 minute more. While the skillet heats, pat the steak dry again.
4. If the steak has a fat cap, such as a strip steak or picanha, hold it fat-side down with tongs and sear for 60 to 90 seconds, until the fat cap is well browned. Then lay the steak flat in the pan and cook, flipping every 30 seconds.
5. After about 2 minutes, start checking the internal temperature. Insert a meat thermometer into the thickest part of the steak, keeping it away from the bone and any large pockets of fat. Remove the steak from the pan when it reaches the pull temperature for your preferred doneness: 110°F (43°C) for rare, 120°F (49°C) for medium-rare, 130°F (54°C) for medium, 140°F (60°C) for medium-well, or 150°F (66°C) for well-done. See the complete steak temperature chart above the recipe card.
6. Transfer the steak to a cutting board and let it rest for 5 minutes.

Assemble the Steak Sandwiches

1. Once the steak has rested, slice it very thinly. If you’re using hanger, skirt, flank, or bavette steak, slice it against the grain. Not sure how? Check out my guide to slicing steak against the grain.
2. Split the ciabatta rolls lengthwise and place them cut-side up on a foil-lined baking sheet. Spread chimichurri over the bottom halves, add the sliced steak, spoon a little more chimichurri over the steak, then top with the cheese.
3. Leave the sandwiches open and place the baking sheet under the broiler, close to the heating element. Broil for 3 to 5 minutes, until the cheese melts and the bread is lightly toasted.
4. Remove from the oven, add the arugula, spread a little more chimichurri over the top halves, then close the sandwiches. Serve immediately while warm.