Preheat the oven to 190°C, fan 170°C, gas 5.
Pat the chicken dry with kitchen towel and place in a roasting tin. Stuff the onion and garlic into the cavity. Rub the oil all over the chicken and season with salt and pepper. Cook according to the instructions on the pack, basting once or twice during cooking to moisten.
Fifteen minutes before the end of the roasting time, put the potatoes in a pan of boiling salted water and cook for 15 minutes.
Remove the chicken from the tin and place on a serving dish. Cover with foil and set aside. Spoon off the fat from the roasting tin, leaving behind the meat juices. Place the roasting tin over a medium heat on the hob and pour in the stock. Mix well, then bring to the boil. Strain the liquid into a clean saucepan and simmer until reduced by about a third.
Add the watercress to the liquid and cook gently until just wilted. Using an electric hand blender, blitz the liquid until smooth. Stir in the lemon rind, dill and cream and reheat gently. Season to taste with salt and pepper.
Cook the green beans in boiling salted water for 3 minutes, until just tender. Drain the beans and potatoes. Slice the chicken and serve with the watercress sauce, potatoes and beans.