1. Preheat the oven to 200°C/400°F/gas mark 6.
2. Mix into a bowl the onion, breadcrumbs, minced lamb, chestnuts, cranberries and walnuts. Season well with salt and plenty of black pepper.
3. Make nicks in each rack of lamb, between each chop, on the opposite side to the fat of the chops. The cuts should be only 1–2cm deep. Stand both racks of lamb upright, bones in the air, fat sides together. Curve them around; the nicks you made should open slightly to allow you to form a circle from the two racks, fat-side inwards. Tie with kitchen string, fairly firmly. Pile the stuffing in the centre, packing it down well.
4. Cook in the oven for 30 minutes, then remove and rest for 10 minutes before carving.
5. Blanch the French beans for 2 to 3 minutes in salted water and drain. In a frying pan, melt the butter until foaming, toss the beans with the chestnuts, walnuts and cranberries. Season to taste. Carve the lamb and serve with the winter green beans.