Preheat oven to Gas Mark 4, 180°C (350°F).
Prepare the marinade by mixing 1 tablespoon olive oil, lemon zest and juice, garlic, rosemary and black pepper. Place the chicken in a large dish and brush over the marinade. Marinate in the fridge overnight if possible.
To cook: Place the potatoes, onion, rosemary leaves, butter, and seasoning in a large bowl and mix well. Spread them over the base of a large roasting tin. Pour over the stock or water. Place the chicken on top, skin side up. Season with salt. Drizzle the remaining ½ tablespoon oil over the chicken.
Roast in the preheated oven, uncovered, for 1 hour or until the juices run clear. Remove from the oven, cover with foil and allow to rest 10 minutes
While the chicken is cooking combine all the ingredients for the dressing. Taste and adjust the seasoning if necessary.
To serve: Quarter or carve the chicken and drizzle over some dressing. Serve with the potatoes, some tender stem broccoli and remaining dressing.