Preheat oven to 200ºC. Score the beef in fine parallel lines, then rub with mustard, Herbes de Provence, garlic and 1 Tbsp extra virgin olive oil, then season with salt and pepper.
Place the celery in the bottom of a roasting pan and set the beef on top. Bake for 40 minutes. Add the stock, reduce the heat to 160ºC and bake for 35 more minutes, until medium-rare.
Pour the remaining oil into a large saucepan set over a high heat and fry the potatoes and swedes for 5 minutes. Add the butter and continue cooking for 15 minutes, stirring constantly, until soft and golden. Serve with the carved beef, minted peas and pickled onions.