Mince the garlic and then mash into a paste with the kosher salt using the flat side of a large chef’s knife.
Stir together the spices in a bowl and then stir in the garlic and oil until a paste forms. Pat the flank steak dry and rub the paste onto both sides of the steak. Place the steak in a large zip lock bag and let marinate for about 6 hours in the refrigerator, turning occasionally.
Bring the steak to room temperature, about 30 minutes. Prepare the grill.
Grill the flank steak on a lightly oiled grill rack, turning once, about 10 minutes per side. Transfer the steak to a platter and tent with foil to keep warm; let rest for 5 minutes before serving.
Slice the steak against the grain into thin slices. Serve.