
Line a large rimmed baking sheet with baking paper (for easier clean up). Place the chicken pieces on the tray. Slice 2 lemons into rounds about 5mm (1/4 inch) and place on the tray. Save the other lemon for making the sauce. Cut each onion (if using) lengthwise into 8 wedges, discarding the skin. Place them on the tray as well.
In a small bowl mix 2 tablespoons of the za'atar (save the rest for the sauce), all spice (if using), 1/2 teaspoon salt and 2 tablespoons olive oil. Drizzle the spiced oil over the chicken, smearing it with the back of a spoon to coat as much of the chicken as you can.