Preheat the oven to 180° C/ 356° F, on convection mode.
In a bowl, mix 200 g (7 oz) of butter, rosemary, salt, and pepper thoroughly, wearing disposable gloves.
Place the butter mixture under the skin of the chicken. This is done by gently separating the skin from the meat (without removing it entirely) to create a pocket between the two. The herbed butter mixture is then spread directly onto the meat beneath the skin.This technique helps keep the meat moist and infuses it with the flavors of the butter, rosemary, salt, and pepper during roasting. Season the inside of the chicken with salt and pepper, then stuff it with the pine nuts, prunes, and the remaining butter. Tie the legs with kitchen twine.
In a roasting pan, spread out the onions, carrots, celery, bay leaves, and peppercorns, and pour in the water.
Place a rack over the pan and put the chicken on top. Cover it well with parchment paper and aluminum foil and roast for 1 hour.
After, uncover the chicken and continue roasting for an additional 20 minutes, until the skin turns golden brown.
Transfer the contents of the pan to a saucepan and blend with an immersion blender until it becomes a thick sauce.
Place a fine-mesh strainer over a bowl and strain the sauce to achieve a smooth texture.
Return the sauce to the saucepan and simmer until it thickens.
Serve the chicken hot, drizzled with the sauce.