Pat the flap steak dry
Season both sides with salt and pepper
Let it sit at room temperature for 20 to 30 minutes
Preheat a grill, cast-iron skillet, or broiler to high heat
Lightly oil the cooking surface or the steak
Cook the steak for 3 to 5 minutes per side for medium-rare, depending on thickness
Flip only once if possible
Check for an internal temperature of 130 to 135°F for medium-rare
Remove the steak from the heat
Rest it for 5 to 10 minutes
Slice thinly against the grain
Serve immediately