Sprinkle the roast beef generously on all sides with salt and coat with the pepper. Put the roast on a rack in a roasting pan. Arrange the garlic halves in a single layer of foil with some foil to spare. Drizzle with olive oil and sprinkle with salt. Wrap the foil over the garlic into a little package and place in the oven. Roast 45 minutes, then set aside.
Meanwhile, place the roasting pan in the center of the oven and cook, undisturbed, for 20 minutes. Lower the temperature to 350 degrees F, and continue to cook until a thermometer inserted in the center of the meat registers between 135 and 140 degrees F for medium-rare, about 2 hours Remove the roast beef and place it on a cutting board to rest for 20 minutes. (The roast temperature will continue to increase as it rests; the end slices will be more well done.)
For the gravy: Pour off excess grease from the roasting pan, then add the wine and bring to a boil over a burner. Reduce the liquid by half and whisk in the mustard. Add the stock and simmer until the sauce thickens slightly, 8 to 10 minutes. Add the lemon juice. Season to taste.
Squeeze 4 of the garlic halves like lemons over the blender to release the flesh from the skins, allowing the flesh to fall into the blender. Ladle some of the gravy into the blender and blend until smooth. Whisk the garlic back into the gravy. Squeeze the remaining garlic halves directly into the gravy and use the back of a spoon to smash them into the gravy. Taste for seasoning.
Serve the roast whole on the platter to carve at the table with the gravy on the side, or arrange slices of beef on a platter and coat with a layer of the gravy.
This recipe has been updated and may differ from what was originally published or broadcast.