In a small cup, combine a tablespoon of the oil, vinegar, a teaspoon of the garlic, oregano, and 1/4 teaspoon each of the salt and pepper; brush over flank steak and let marinate on a plate at room temperature 10 minutes. Meanwhile, heat a stovetop grill pan over medium heat.
In a small bowl, stir together yogurt, cucumber, mint, and the remaining garlic, salt and pepper until blended.
Grill steak 6 to 7 minutes per side or until the internal temperature registers 140 °F (60 °C) on an instant-read thermometer, for medium-rare. Transfer to a cutting board and let rest 5 minutes.
While steak is grilling, heat remaining oil in a large non-stick skillet over medium-high heat. Add peppers and onions and sauté 8 minutes or until crisp-tender.
Thinly slice steak across the grain. Top each wrap with 2 lettuce leaves, steak, peppers, onions, and some yogurt sauce. Roll wraps around filling.
Wrap souvlakis in parchment or foil.
There are 325 calories in 1 serving of Flank Steak Souvlakis.