Preheat oven to 425 degrees. Line 2 baking sheets with foil and set aside.
In a medium bowl, whisk together 3 Tablespoons Olive Oil with sumac, salt, brown sugar, paprika and basil. (If mixture is too paste like, add a bit more olive oil.)
In a large mixing bowl, toss the carrots and cauliflower with halve of the spice mixture. Transfer the vegetables to the baking sheets.
In the same large mixing bowl, rub the chicken with the remaining spice mixture. Transfer chicken to the baking sheets and top with lemon slices.
Roast chicken and vegetables in the oven for 15 minutes. Then add the onions slices and toss vegetables. Cook an additional 15 to 20 minutes until vegetables are fork tender and the internal temperature of the chicken is 165 degrees.
Mix together remaining 3 tablespoons olive oil, parsley, cilantro, lemon juice and garlic in a small bowl. Spoon mixture over cooked chicken and vegetables and serve immediately.