Position an oven rack 6 inches below the broiler unit (so not directly below). Preheat the broiler to high (550°F).
Line a large, rimmed baking sheet with foil. Place it in the oven and allow it to heat up for about 5 minutes while you prepare the meat for broiling.
In a small bowl, stir together the olive oil, minced garlic, kosher salt, black pepper, and red pepper flakes. Set the mixture aside.
Unfold the steak and place it on a cutting board or another work surface. Examine it and locate the meat fibers - later, when it's cooked, you'll want to cut across them.
Rub the steak all over with the olive oil rub.
Carefully, using oven mitts, remove the hot pan from the oven. Place the steak in the pan. Place the pan in the oven and broil until the steak is browned, about 8 minutes for medium-rare.*
Remove the steak to a cutting board. Loosely cover it with foil and let it rest for 10 minutes.
Slice the steak thinly against the grain, as shown in the video below. Serve immediately.
Video
Notes
*The USDA says that whole cuts of beef should be cooked to an internal temperature of 145°F (medium) with a 3-minute rest.
Like all steaks and roasts, it's important to let the meat rest before you slice it. If you cut into it right away, juices will escape into the cutting board, and the steak will not be as juicy.
Cutting against the grain means cutting a piece of meat in a way that severs its muscle fibers, making it easier to chew and digest. It's best to examine the steak when it's still raw - that's when it's easiest to identify the direction of the grain. You'll basically be looking for parallel lines of muscle fiber running down meat. In a flank steak, the muscle fibers are prominent and easy to spot, and they also tend to all run in the same direction, so they are easier to sever.
You can keep the leftovers in the fridge, in an airtight container, for up to 4 days. You can gently reheat them, covered, in the microwave at 50% power, or chop them cold and add them to a salad.