To cut a pocket into the flank steak, slice an opening on the longer and thicker side of the steak. Turn the steak over and slice deeper into the meat, stopping 1/2" from the three other edges. Place the steak in a 9" x 13" baking dish.
To make the marinade, combine the olive oil, vinegar, honey, and rosemary in a bowl and pour it over the steak. Let the steak marinate for 30 minutes.
Preheat the grill to high heat.
To make the stuffing, combine the feta, garlic, bell pepper, scallions, salt, and black pepper in a bowl.
Remove the steak from the marinade. Discard the marinade. Combine the salt and pepper and season the steak. Fill the pocket with the stuffing mixture. To keep the filling from falling out, weave a metal or wooden skewer (soaked in water for 30 minutes) in and out of the steak's opening. Grill the steak for 12–14 minutes on each side for medium-rare, or longer if you prefer the steak medium or well-done. Remove the steak to a cutting board and let it stand for 5–10 minutes before thinly slicing on the bias.
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