A rib roast is an investment - take care to rest the meat well to make the most of it.
2 tbspolive oil2 tbspchopped rosemary2 tspDijon mustard, plus extra to serve3anchovy fillets, crushed with a mortar and pestle1rack of beef (also known as a standing rib roast), with at least 5 ribs (4.3kg)3onions, thickly slicedPotato gratin500 mlpouring cream4garlic cloves, crushed5Desiree potatoes (about 2kg)½nutmeg
Combine oil, rosemary, mustard and anchovies in a non-reactive dish, add beef, turn to coat, cover and marinate at room temperature (1 hour).
For potato gratin, preheat oven to 190C. Whisk cream and garlic in a bowl to combine. Peel and thinly slice potatoes on a mandolin into the cream, tossing to coat as you go to prevent discolouration, season to taste and arrange in even layers in a buttered 30cm x 30cm oven dish. Season with freshly ground black pepper and a grating of nutmeg, cover with baking paper then foil and bake on lowest rung of oven until a knife pierces potato easily (45 minutes to 1 hour). Uncover and cook until golden (30 minutes). Keep warm.
Meanwhile, place beef on top of onions in a roasting dish and roast on top shelf until medium-rare (1½ hours). Set beef aside loosely covered with foil to rest (30 minutes), then serve with gratin and mustard.