FOR THE VINAIGRETTE
8 ounces scallions (about 3 bunches)
1/2 cup plus 2 tablespoons champagne vinegar (or white wine vinegar)
2 teaspoons kosher salt
1 teaspoon crushed red pepper flakes, ground in a spice grinder or mortar and pestle
1 cup extra-virgin olive oil
FOR THE STEAK
6 medium or large garlic cloves
1 tablespoon kosher salt, or more as needed
1 tablespoon freshly ground black pepper
4 pounds skirt steak
1/4 cup olive oil, plus more for drizzling and as needed
Finishing-quality extra-virgin olive oil
For the vinaigrette: Trim off and discard the root ends and any wilted dark greens from the scallions. Finely chop the scallions and put them in a bowl. Add the vinegar, salt and ground red pepper flakes. Whisk in the extra-virgin olive oil in a slow, steady stream to create a thick vinaigrette. The yield is about 2 cups.
For the steak: Pulse the garlic (to taste) in a mini food processor to mince it. Add the kosher salt and pulse again just to combine. Alternatively, put the garlic in a small mortar; sprinkle with the kosher salt and pound to break up the garlic. Or mince it by hand, adding the kosher salt midway through mincing to help the garlic break down. Stir in the black pepper.
Lay the skirt steaks on a flat work surface. Spoon the garlic rub onto the steaks, using about 1 tablespoon of rub per pound of meat, and use your clean hands to rub it all over, putting more rub on the thickest parts of the steak. Turn the steaks to coat the other sides with the rub. Let them sit for 30 minutes, or cover and refrigerate for as long as overnight. Bring the steaks to room temperature before cooking them.
Cut the skirt steaks into pieces that will fit in the pan you’re cooking them in, as needed.
Heat 2 tablespoons of the olive oil in a large grill pan or cast-iron skillet over high heat until it slides easily in the pan and the oil around the edges of the pan just begins to smoke, 2 to 3 minutes. Drizzle a thin layer of the olive oil on both sides of the steak. Working in batches, lay the skirt steaks in the pan in a single layer and sear each side until it is deep brown and caramelized, 3 to 4 minutes per side for medium-rare. Transfer to a baking sheet or plate to rest while you sear the remaining steaks.
Add the remaining 2 tablespoons of olive oil to the pan and heat for about 1 minute, until it starts to smoke around the edges of the pan, before adding the remaining steaks and searing them in the same way.
To serve, transfer the steaks to a cutting board, preferably one that has a moat to catch the juices. Slice the steaks against the grain 1/2 inch thick on the diagonal. Drizzle the meat with finishing-quality extra-virgin olive oil, sprinkle with salt, and pass the scallion vinaigrette at the table.