Preheat a charcoal griller (such as Big Green Egg(R)) to 375 degrees F (175 degrees C) and lightly oil the grate.
Lay flank steak on a flat work surface. Cut steak from one side through the middle horizontally to within 1/2 inch of the other side. Open the two sides and spread them out like an open book.
Arrange butcher's twine on a plate, enough to cover the length of the steak. Place each string about 1 inch 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 twine.
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 140 degrees F (60 degrees C).