How to Make It
Preheat the oven to 425°. Wrap the garlic heads in a double layer of foil and bake for about 1 hour, or until soft. Lower the oven temperature to 350°.
Peel the roasted garlic. In a small bowl, mash the garlic cloves with a fork and mix in the mustard.
Toast the bread in the oven for 6 to 7 minutes, or until dry. Let cool, then break into pieces and coarsely chop in a food processor. You should have 1/2 cup of crumbs. Add the bread crumbs to the garlic and mustard mixture. Raise the oven temperature to 425°.
Heat the oil in a large, heavy roasting pan set over 2 burners on moderate heat until almost smoking. Season the tenderloin with salt and pepper. Add the tenderloin to the roasting pan and brown well on all sides, about 15 minutes total. Transfer to a platter to cool.
Pour off the fat from the roasting pan. Add the red wine to the pan and bring to a boil over moderately high heat, scraping up any browned bits from the bottom. Boil the wine until reduced by half, about 3 minutes. Add the wine to the reserved vegetable cooking liquid from the Winter Vegetable Pan-Roast.
Spread the garlic mixture all over the top and sides of the tenderloin. Press the thyme into the garlic mixture. Transfer the tenderloin to the roasting pan and roast for about 25 minutes, or until the crust is browned and an instant-read thermometer inserted in the center of the meat registers 125° for medium rare. Transfer to a cutting board and let rest for 15 minutes.
Set the roasting pan over 2 burners on moderately high heat. Pour in the vegetable cooking liquid and boil for 2 minutes, stirring with a wooden spoon. Pour the liquid into a medium saucepan and bring to a boil over moderate heat. Whisk in the flour paste until smooth and simmer, whisking frequently, until the gravy thickens, about 3 minutes. Season with salt and pepper and strain into a warmed gravy boat. Carve the roast into 1/3-inch-thick slices and pass the gravy at the table.