Preheat the oven to 200°C/400ºF/gas 6 and get your oven racks into position – one in the middle and one just above.
Slice the potatoes in half lengthways and slice the tops off the garlic bulbs, then place in a shallow roasting tray. Season generously with sea salt and black pepper and drizzle with 1 teaspoon of olive oil, then set aside.
Using a large sharp knife, carefully cut down the back of the chicken and open it out flat. Rub the butter and a good pinch of salt and pepper all over both sides.
Place the potato tray on the bottom shelf of the oven, and the chicken directly onto the shelf above. Place the unpeeled onions on top of the chicken and roast for 1 hour 10 minutes, turning the potatoes regularly and brushing the chicken with the cooking juices using the rosemary sprigs.
Trim the beans and cook in boiling salted water for 5 minutes, or until tender. Finely chop the walnuts.
Spoon the mustard into a bowl, peel and finely grate in the garlic, then whisk in 1 tablespoon each of walnut oil and extra virgin olive oil and a dash of white wine vinegar. Season to perfection with salt and pepper.
Drain the beans and toss them in the dressing, then tip onto a serving plate and sprinkle with the walnuts.
Remove the chicken from the oven and leave to rest for a few minutes. Squash the onions and garlic out of their skins and toss through the potatoes. Serve with the chicken and green beans.
If your chicken comes with giblets, use them to make this delicious Hot pâté toast