Pat the chicken skin dry with paper towel and generously season with sea salt flakes and pepper all over, not forgetting the cavity. If time allows, allow the chicken to come to room temperature for 2 hours. This step acts like a dry brine for juicy chicken results.
Preheat the oven to 170°C conventional (150°C fan-forced). Using your fingers, carefully separate the skin from the chicken breast and evenly stuff with 30 g of the butter. Season the chicken with garlic powder and fennel seeds and extra salt and pepper. Drizzle the olive oil all over and massage well into the chicken with your hands.
Using a sharp knife, slice the top off the garlic bulb, about 2–3 cm down from the top, to expose the cloves, then place in the cavity of the chicken with the lemon and fresh herbs. Tie the legs of the chicken together with kitchen string as this will help the chicken cook evenly.
Transfer the chicken to a baking dish with the leek. Season the leek with salt, pepper and then the remaining 20 g butter. Bake for 1 hour and 20 minutes, or until the chicken is cooked through and golden brown.
Meanwhile, prepare the herb sauce by placing the garlic and a big pinch of sea salt to a mortar and pestle. Grind until a paste-like texture forms. Add the anchovies and grind them into the garlic paste. Add the capers and pound until they are slightly broken down. Add the parsley and mix them through.
Pour the olive oil and lemon juice into the mortar and pestle and gently mix through until all the ingredients are well combined. Taste and adjust seasoning to your liking.
Allow the chicken to rest for 10 minutes before carving. Serve with salsa verde and sourdough.