Preheat oven to 425 degrees F. Line 2 baking sheets with foil and set aside.
In a medium bowl whisk together three tablespoons olive oil with the sumac, salt, brown sugar, dried basil, and paprika.
In a large mixing bowl toss the carrots and the cauliflower with half the olive oil and spice mixture. Transfer the vegetables to the prepared baking sheets, dividing them between the two sheets.
In the same large mixing bowl, rub the chicken with the remaining olive oil and spice mixture. Transfer chicken and lemon slices to the baking sheets and arrange among the carrots and cauliflower.
Transfer baking sheets to the oven and bake for 15 minutes. After 15 minutes toss vegetables, add the onion to the baking sheets and rotate pans. Cook for an additional 15-20 minutes, or until internal temperature reads 165 degrees F.
Mix together the remaining 3 tablespoons olive oil, parsley, cilantro, lemon juice and minced garlic in a small bowl. Spoon mixture over cooked chicken and vegetables and serve immediately.