Preheat a hooded charcoal barbecue to 175 degrees C and lightly oil the grill plate.
Lay flank steak on a flat work surface. Cut steak from one side through the middle horizontally to within 1cm of the other side. Open the two sides and spread them out like an open book.
Arrange butcher's string on a plate, enough to cover the length of the steak. Place each string about 3cm apart. Place steak on top; season with salt and pepper. Lay down a thick bed of spinach on top. Scatter feta cheese, olives and bacon over the spinach.
Roll steak up starting from the edge closest to you, pressing filling down to tighten the roll. Tie with butcher's string.
Cook over direct heat, turning at least once, until steak is beginning to firm and is hot and slightly pink in the center, 6 to 7 minutes per side. An instant-read thermometer inserted into the center should read 60 degrees C. Stand, loosely covered, for 10 minutes before slicing.