Preheat a grill to medium high. Make the marinade: Whisk the olive oil, garlic, mint (if using), oregano, paprika, 1 teaspoon salt, and pepper to taste in a large bowl. Transfer 1 tablespoon of the marinade to a small bowl; mix in the yogurt and 2 tablespoons water. Add the bell pepper and onion to the bowl with the remaining marinade and toss; remove to a plate with a slotted spoon. Add the steak to the bowl and toss.
Grill the bell pepper, onion and steak, turning once, until lightly charred, about 4 minutes per side for the vegetables and 4 to 8 minutes per side for the steak (depending on the thickness). Set aside to rest, about 5 minutes. Meanwhile, grill the pitas until marked, about 1 minute per side.
Slice the steak against the grain. Fill the pitas with the steak, grilled vegetables, tomato and watercress. Drizzle with the yogurt sauce.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine