Heat oven to 425F (220C). Line a rimmed baking sheet with a roasting rack; set aside.
Place basil, parsley, pecorino, walnuts, garlic, lemon zest, lemon juice, olive oil, salt and pepper in the bowl of a food processor or in a blender and pulse until well combined.
Spatchcock the chicken by removing the backbone. To remove, flip the chicken over so that it sits breast side down. Use a pair of kitchen shears to cut along either side of the backbone. Flip the bird over and press down flatten. (Discard backbone or reserve for stock.)
Place the chicken breast side down on roasting rack and rub 2 tablespoons (30g) of pesto over the underside of the chicken. Flip the chicken over, separate the breast skin from the meat with a spoon or hands and spoon two tablespoons of pesto under the skin and evenly distribute it as best you can. Coat outside of the chicken with another 3 tablespoons (45g) of pesto and season with salt and pepper.
Roast for 20 minutes. Turn the heat down to 350F (176C) and continue roasting for another 25 to 35 minutes or until the internal temperature reaches 165F (74C).
Remove from oven, tent with aluminum foil and allow it to rest for 10 minutes prior to slicing to serve or cool completely to use on the Roasted Panzanella or the Pesto Chicken BLT, saving remaining pesto as well.