Combine seasoning, cooking oil, garlic, and lemon juice in a large bowl or plastic bag.
Add chicken and toss to coat.
Refrigerate for at least 1 hour and up to 8 hours. (Note: If you’d like to prepare the marinade even further ahead, you can refrigerate the chicken in marinade for up to 3 days, but only add the lemon juice in the last 8 hours. Lemon juice will make the chicken tough and rubbery if left to marinate for more than 8 hours.)
When ready to cook, preheat oven to 425°F. Line a sheet pan with foil (not necessary, but makes clean-up much easier).
Add onion to chicken and marinade and toss everything together.
Pour chicken, marinade, and onion out onto prepared sheet pan. Season the chicken with salt and pepper.
Transfer chicken to oven and roast until chicken is golden brown on the edges and cooked through (use an instant read thermometer to make sure it registers an internal temperature of 165°F), 30 to 40 minutes.
Transfer chicken and onions to a cutting board, leaving the sheet pan with all of the cooking liquids nearby. Roughly chop chicken and onions together and add them back to the sheet pan. Toss chicken back in cooking liquids.
Scatter parsley (if using) over top of chicken.
Warm pita / naan in the oven.
Serve with pita / naan, cucumber and tomato salad, tzatziki, hummus, and carrot sticks.