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.