Cut the flank steak against the grain into slices of about 1/4-inch thick, or as thin as you can. One by one, place the slices between two sheets of parchment or wax paper. Use a meat pounder to pound them very thin, less than 1/8-inch, or until you start to see the bottom sheet of paper through the meat (but not so thin that it is completely breaking apart).
As you move along, place the pounded slices on a cooling rack set on top of a large sheet pan or chopping board. Once done, sprinkle 1 1/2 teaspoons salt on one side. Flip over and sprinkle with remaining 1 1/2 teaspoons salt.
Leave to air dry and cure for at least 3 hours and up to 8 hours. (Alternately, you can leave at room temperature for 2 hours and refrigerate for the remaining time.)
When ready, pre-heat a grill or grill pan over medium-high heat. Once hot, brush the meat with vegetable oil and cook in batches for 1 to 2 minutes per side. Rest under aluminum foil until ready to use.