Sprinkle the sliced or chopped onions on a sheet pan then place the chicken in the center. Lay out the halved lemons and garlic face up all over the sheet pan then drizzle olive oil over everything and sprinkle with salt and pepper.
Cover half of the chicken with the sumac and the other half with za’atar. Don’t mix them or you’ll ruin the look! Squeeze 1-2 of the lemon sliced over the top of the chicken.