Preheat the oven to 350°F (175°C).
Prepare the gremolata by combining the chopped parsley, cilantro, lemon juice, lemon zest, and minced garlic in a small bowl. Mix well and set aside.
Arrange the chicken thighs, 8 garlic cloves, and mixed olives in a large baking dish.
In a small bowl, stir together the harissa paste, tomato paste, brown sugar, and chicken stock until combined.
Spoon the harissa mixture evenly over the chicken, garlic, and olives. Season with salt and cracked black pepper.
Cover the baking dish tightly with foil and roast for 1 hour.
Increase the oven temperature to 400°F (200°C). Carefully remove the foil and arrange the Roma tomatoes around the chicken.
Return the uncovered Harissa Chicken Thighs to the oven and roast for another 30 minutes, or until the chicken is fully cooked and the tomatoes are tender and lightly caramelized.
Check that the thickest part of the chicken has reached an internal temperature of at least 165°F (74°C), avoiding contact with the bone when measuring.
Spoon some of the tomato-harissa sauce over the chicken and finish generously with the prepared gremolata before serving.