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, garlic cloves, and mixed olives in a large baking dish.

In a small bowl, combine the harissa paste, tomato paste, brown sugar, and chicken stock. Stir until well blended.

Spoon the harissa mixture evenly over the chicken, garlic, and olives. Season everything with salt and cracked black pepper.

Cover the baking dish tightly with foil. Roast the Harissa Chicken Thighs for 1 hour.

Increase the oven temperature to 400°F (200°C). Carefully remove the foil and add the Roma tomatoes around the chicken.

Return the uncovered dish to the oven and roast for another 30 minutes, or until the chicken is fully cooked and tender and the tomatoes have softened.

Spoon some of the pan sauce over the chicken and finish with the fresh gremolata before serving.