Pat the flank steak thoroughly dry with paper towels to ensure a crisp sear. Season both sides generously with salt or your choice of spices.

Allow the steak to sit at room temperature for 30 minutes. This resting period is essential for taking the chill off the meat, allowing the muscle fibers to relax so the steak cooks evenly from edge to center.

Place a large cast-iron skillet or heavy-gauge pan over medium-high to high heat. Add the beef tallow to the pan and allow it to melt until it is shimmering and just beginning to smoke.

Carefully place the flank steak into the hot pan. Press down slightly to ensure full contact with the surface. Cook the steak for 4 to 6 minutes on the first side without moving it, allowing a dark, flavorful crust to develop.

Flip the steak and sear the second side for an additional 4 to 6 minutes for a medium-rare finish. Because flank steak is relatively thin, monitor the internal temperature closely to avoid overcooking.

Remove the steak from the pan and transfer it to a clean cutting board. Let the meat rest undisturbed for 5 to 10 minutes to allow the internal juices to redistribute.

Using a sharp knife, slice the steak into thin strips against the grain, holding your knife at a slight diagonal. Serve warm. Leftovers can be stored in an airtight container in the refrigerator for up to 4 days and are excellent served cold over a salad or gently reheated in a pan.