How to Make It
Prepare an outdoor grill (preferably charcoal) to medium-high heat.
Whisk together all ingredients in a small bowl. Set aside.
Combine lamb, onion, parsley, mint, za’atar, oil, salt, pepper, and garlic in a medium bowl. Mix well with your hands to combine. Spread one-fourth of lamb mixture in a thin layer over inside surface of one pita half (hands work best for this; spread all the way to the edge). Top lamb with secondvpita half. Repeat process with remaining lamb mixture and remaining 3 pitas.
Arrange filled pitas on oiled grill grate. Cover and grill until pitas are toasted and meat is cooked through, 4 to 5 minutes, turning once halfway through cook time. Cut into wedges.
Combine watercress and radishes in a large bowl. Drizzle with oil and lemon juice, and sprinkle with salt and pepper; toss gently to coat. Serve salad with pitas and sauce.