Wash and dry produce.

Quarter tomato lengthwise; cut crosswise into ¼-inch-thick pieces. Halve cucumber lengthwise; slice crosswise into ¼-inch-thick half-moons.

Pat chicken* dry with paper towels; season all over with half the Za'atar Spice (you'll use the rest later), salt, and pepper. Heat a drizzle of oil in a large pan over medium-high heat. Add chicken; cook until browned and cooked through, 5-7 minutes per side.

Meanwhile, in a small bowl, combine mayonnaise, garlic powder, half the onion powder (all for 4 servings), salt, and pepper.

Halve and toast ciabattas.

In a large bowl, whisk together vinegar, mustard, remaining Za'atar Spice, a large drizzle of olive oil, a pinch of sugar, salt, and pepper.

Add mixed greens, tomato, and cucumber; toss until coated.

Evenly spread cut sides of ciabattas with garlic onion mayo. Top bottom halves of ciabattas with chicken and as much salad as you like. Close to form sandwiches.

Divide sandwiches and any remaining salad between plates. Serve.