For the meat:
5-6 pounds beef tenderloin, trimmed
3 teaspoons coarse salt
1 1/2 teaspoons fresh cracked black pepper
2 tablespoons canola or vegetable oil
1/2 cup unsalted butter, divided
4 large cloves garlic, minced
1/2 teaspoon Dijon mustard, optional
For the sauce:
2 tablespoons vegetable or canola oil
1 onion (or brown shallots), finely chopped
1 1/2 cups dry red wine
2 cups beef stock or broth
1 sprig of fresh thyme or rosemary
1/8 teaspoon ground black pepper
1/4 teaspoon salt
Slice the tenderloin in half crosswise to create 2 smaller pieces that are equal in size and thickness. If the roast is untied, fold the ends up against the thicker/center section of each roast and tie with kitchen twine spaced about 1 inch apart.
Chill in the refrigerator for 12 hours, uncovered. Then allow the beef to stand at room temperature for 1-2 hours before roasting.
Place the oven rack in the middle position and preheat to 430 degrees F.
Season each beef tenderloin thoroughly with salt and pepper.
Heat the oil in an ovenproof skillet over medium-high heat until hot. Sear the roasts for 3 minutes on each side, for all four sides, turning over with the tongs until nicely browned.
Combine the butter, Dijon mustard, and garlic in a small bowl while the roasts are searing. Set half of the butter mixture aside to use later for the sauce. Coat the remaining garlic butter over each roast (about 2 tablespoons each).
Move the skillet into the hot oven. Roast each tenderloin for 20-25 minutes, or until an instant-read thermometer inserted into the thickest part of the roast reads 120-125 degrees F for medium-rare. (Rare: 115-120 degrees F or Medium: 130-135 degrees F)
Move the roast to a carving board and let sit for 15 minutes. Retain the juices in the pan for the sauce. Carve the beef tenderloin into 1/2-inch thick slices and serve with the red wine sauce.
To prepare the red wine sauce, skim the fat off of the remaining juices in the pan. Heat the oil in the pan with the leftover juices over medium-high heat.
Sauté the onion for 3 minutes, or until softened and lightly browned, scraping up the leftover browned bits.
Add in the rosemary/thyme and season with pepper. Cook for an additional minute while continuously stirring. Pour in the red wine and cook until reduced by half.
Add in the beef stock and bring to a boil. Reduce the heat and simmer until reduced by half. Remove the rosemary/thyme sprig.
Season with salt to taste, then whisk in the reserved garlic butter. Pour in any of the remaining juices from the resting steaks immediately before serving.