Preheat the oven to 425°F. Line a large baking sheet with parchment paper for easy cleanup.
In a large bowl, combine the chicken thighs, salt, black pepper, oregano, garlic powder, and olive oil. Toss until the chicken is well coated.
Arrange the chicken thighs on one side of the prepared baking sheet. On the other side, add the red onion, bell pepper, zucchini, cherry tomatoes, and lemon slices.
Roast in the preheated oven for 35-40 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F. The vegetables should be tender and slightly caramelized.
Remove from the oven and let it rest for 5 minutes. Garnish with fresh parsley before serving.