1Take the beef out of the fridge 1hr before roasting, to bring it up to room temperature.
2Preheat the oven to 240C/220C Fan/Gas 9.
3Remove the netting from the beef. With a sharp knife, cut small slits in the meat; fill each with a little rosemary followed by a small peeled clove of garlic.
4Place the peppercorns, juniper berries, 2 sprigs of the rosemary and the 3 chopped cloves garlic in a food processor. Pulse into a rough paste. Alternatively, grind together in a pestle and mortar.
5Rub the paste all over the beef, then drizzle over ½ the oil. Place in a roasting dish a little larger than the beef joint. Surround with any garlic left over from the heads, plus the shallots, whole thyme sprigs, 3 of the bay leaves, and the remaining rosemary.
6Put the beef into the oven and turn down the heat to 220C/200C Fan/Gas 7. Roast for 1 hr, occasionally basting with the juices, for a medium roast; add 10 mins for well done; subtract 10 mins for rare. Cover with foil after 40 mins if it’s turning too dark.
7Remove the beef from the oven, cover with foil and set aside to rest for 15 mins.
8Meanwhile, make the mash. Put the milk, butter, nutmeg, crushed garlic and 2 remaining bay leaves in a small pan. Bring to a boil, then immediately remove from the heat. Set aside to infuse for 30 mins, then strain; discard the garlic and bay leaves.
9While the milk is infusing, heat the remaining oil inafrying pan on a medium setting. Add the bacon and mushrooms; fry for 10 mins until crisp and golden. Set ½ aside.
10Add the stock to the pan, bring to a boil, and simmer for 10 mins. Remove from the heat; stir through the crème fraîche and the thyme leaves. Season with black pepper. Keep warm on a low heat, stirring occasionally.
11Bring a large pan of water to the boil. Add the potatoes and simmer for 15-20 mins until tender. Drain and leave to steam dry for 30 secs, then return to the pan and mash until smooth.
12Add the infused milk to the mash, beating it in with a wooden spoon until smooth and fluffy. Fold the rest of the bacon and mushroom mixture through the mash; season with black pepper.
13Transfer the beef, shallots, garlic and herbs to a platter and garnish with the extra bay leaves, if using. Carve to serve, with the mash and the sauce on the side.