Preheat a charcoal grill to 375 degrees F (175 degrees C) and lightly oil the grate.

Lay flank steak on a flat work surface.

Cut steak horizontally from one side to within 1/2 inch of the other side, creating a pocket.

Open the steak like a book.

Arrange butcher's twine on a plate, spacing them about 1 inch apart.

Place the steak on top of the butcher's twine.

Season the steak with salt and pepper.

Lay a thick bed of baby spinach on top of the steak.

Scatter crumbled feta cheese, chopped kalamata olives, and cooked crispy bacon over the spinach.

Roll the steak up tightly starting from the edge closest to you, pressing the filling down.

Tie the rolled steak securely with the butcher's twine.

Cook the steak over direct heat, turning at least once, until it is beginning to firm and is hot and slightly pink in the center, about 6-7 minutes per side.

An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C).

Let rest for 10 minutes before slicing and serving.