Mix together the marinade ingredients and toss with the ribs, coating thoroughly. Cover and marinate in the fridge overnight, or for 12 hours.
Preheat the oven to 180C/350F/Gas 4.
To make the celeriac and potato dauphinoise, melt the butter in a large saucepan. Add the garlic and onion and fry gently for three minutes until softened and fragrant.
Add the potato and celeriac. Mix in the cream, season with freshly grated nutmeg, salt and freshly ground pepper and bring to the boil.
Transfer the boiling mixture to a shallow gratin dish. Place the gratin dish in a roasting tray. Pour in hot water so it reaches halfway up the sides of the gratin dish.
Bake in the oven for 30-40 minutes until golden-brown. Set aside then heat through in a warm oven for 10-15 minutes before serving.
Meanwhile, place the ribs in a roasting tray, reserving the marinade. Roast for 40 minutes, turning and basting now and then with reserved marinade. Reduce the heat to 150C/300F/Gas 2 and roast for a further 45 minutes, basting and turning now and then.
Serve the roast ribs with the celeriac and potato dauphinoise and a tomato and green leaf salad.
