For the Sauce

1. Melt 5 tablespoons of the butter in a medium saucepan. Add the shallots and cook over medium-low heat, stirring occasionally, until soft and translucent, 7 to 8 minutes. Add the wine, beef broth, thyme sprigs, salt, pepper and sugar, and bring to a boil. Cook over medium heat for about 30 minutes, or until the liquid is reduced by about half.
2. While the liquid is reducing, place the remaining 3 tablespoons of butter in a small bowl and soften in the microwave, if necessary (it should be soft but not melted). Add the flour and, using a small spoon, mix into a smooth paste.
3. Once the wine mixture is reduced, reduce the heat to low and remove the thyme sprigs. Whisk the flour-butter paste, a tablespoon at a time, into the simmering liquid, and simmer for a few minutes, until the sauce is thickened. Set aside. (The sauce can be made up to this point and refrigerated up to 3 days ahead of time.)

For the Tenderloin

1. Let the beef stand at room temperature for 1 hour before roasting. Set an oven rack in the middle position and preheat the oven to 400°F.
2. Season the beef all over with kosher salt and pepper. Heat the oil in an oven-proof skillet over medium-high heat until almost smoking. Cook, turning with tongs, until well browned on all but one side, about 10 minutes total. Turn the tenderloin so that the un-seared side is down, and transfer the skillet directly to the preheated oven. (If your pan isn't oven-proof, transfer the beef to a lightly oiled roasting pan.) Roast until a thermometer inserted into the center of the meat registers 120°F-125° for medium rare, about 15 minutes, or until done to your liking (115°F-120°F for rare, 130°F-135°F for medium). Keep in mind that these temperatures account for the fact that the temperature will continue to rise about 5 degrees while the meat rests.
3. Transfer the meat to a carving board (preferably with a well for collecting juices) and let it rest, covered loosely with aluminum foil, for 10 to 15 minutes. Place a dishtowel or oven mitt over the handle of the roasting pan to remind yourself that it's hot.
4. Meanwhile, carefully discard the fat from the roasting pan (remember that the handle is hot!). Set the pan on the stovetop and add the ¼ cup of broth. Bring the broth to a boil and, using a wooden spoon, scrape the fond, or brown bits, from the bottom of the pan. Add the flavorful broth to the red wine sauce, and then bring the sauce to a simmer.
5. Carve the tenderloin into ½-inch-thick slices. Serve the beef, passing the red wine sauce at the table.