Preheat your oven to 425°F (220°C).
In a large bowl, combine the olive oil, za'atar, garlic powder, onion powder, salt, black pepper, and lemon juice. Mix well to create a marinade.
Add the chicken thighs to the bowl and coat them thoroughly with the marinade. Let them marinate for at least 30 minutes at room temperature or up to 2 hours in the refrigerator for more flavor.
Place the marinated chicken thighs on a baking sheet lined with parchment paper. Arrange the cherry tomatoes and red onion around the chicken.
Roast in the preheated oven for 35-40 minutes, or until the chicken is cooked through and the skin is crispy. The internal temperature should reach 165°F (74°C).
Remove from the oven and let it rest for 5 minutes. Garnish with fresh parsley before serving.