1. Preheat a grill on medium-low heat. Using a fork, mix together the feta, 1/3 cup warm water, 1/4 teaspoon of the Italian seasoning, the lemon zest, 1/2 teaspoon salt and 1/2 teaspoon pepper in a small mixing bowl until the mixture comes together and the feta is in small chunks. Set aside.
2. Heat the oil in a large nonstick skillet over medium-high heat. Add the sirloin, the remaining 1/4 teaspoon Italian seasoning, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook until the meat is no longer pink and begins to brown, 2 to 3 minutes. Drain any liquid off and set aside.
3. Spread the feta mixture on the pitas and top with the cooked meat. Place a large piece of nonstick foil directly onto the preheated grill. Place the pitas on the foil, cover and grill until the bottoms begin to char and the cheese melts, about 3 minutes. Remove from the heat and sprinkle with the sliced mint, olives, tomatoes and onions. Serve warm.