In a medium bowl, add the chicken, garlic, thyme, oregano, rosemary, 1/2 cup olive oil, and season with salt and pepper. Gently toss to combine. Cover and refrigerate overnight. When you are ready to cook the chicken, remove it from the refrigerator. Transfer the chicken to a small baking sheet or large plate.
Add the fava bean greens, pine nuts, garlic, lemon juice and zest, and olive oil to a food processor. Season with salt and pepper and puree until it is smooth. Transfer the pesto to a small bowl.
In a medium bowl, add the spring onions, carrots, fava beans and 2 tablespoons olive oil. Season with salt and pepper and toss to combine. Set aside.
Preheat the oven to 350°F.
Gently stuff the pesto underneath the skin of the chicken thighs. Heat 2 tablespoons olive oil in a cast iron skillet over medium heat. Add the chicken, skin side down, and cook for 5 minutes. Turn the chicken over and cook for an additional 4 minutes. Remove from the heat.
Add the vegetables with the olive oil to the skillet, scattering them around the chicken thighs. Place in the oven and cook for 1 hour, or until the chicken is cooked through and the vegetables are soft. Carefully remove the skillet from the oven.
Transfer the chicken and vegetables to a large platter. Serve immediately.