Preheat the oven to 180°C.
Using sharp kitchen scissors, remove the backbone of chicken. (Use this for a nice stock). Flatten the chicken by placing it skin-side up on a cutting board and pushing down the breastbone.
Cut the potatoes and onion into rings then lay them in a large, lightly oiled baking tin. Season with salt and pepper. Lay the sprig of rosemary on top.
Mix the butter/margarine with the chopped herbs, lemon zest and garlic, then season with salt and pepper. Rub the chicken with this mixture - all over it, under the skin, then crumble the rest on top of the potatoes.
Transfer the chicken to a wire rack, facing skin-side upwards. Cut one lemon into thin rings and slide a few under the skin. Put the rest in the tin.
Bake in the oven for about 45 minutes until golden.
Drizzle with some lemon juice just before serving.