With paper towels, pat the tenderloin dry. Fold the tapered ends of the tenderloin under itself then secure tightly with cooking twine. Continue to tightly tie the tenderloin in sections so it forms a consecutive shape. This will guarantee even cooking.
In a bowl, combine garlic, rosemary, and oil and mix while slightly crushing the garlic to form a light paste. Evenly coat the tenderloin with this mixture and allow sit at room temperature for 1 hour.
Preheat the oven to 500°F. Evenly season the tenderloin with salt and pepper and place on a roasting rack in a roasting pan. Alternatively, using a sided baking sheet without a roasting rack would work but may cause a faster cooking time and uneven cooking. Roast for 20 minutes then insert a food-grade thermometer into the center of the tenderloin. Remove the roast at 120°F or continue to cook, checking every 5 minutes until temperature reaches 120°F for medium-rare. Transfer the roast to a cutting board and loosely cover with foil. Allow to rest 10 to 15 minutes.
While the tenderloin is roasting, create a red wine shallot reduction, if desired. In a saucepan, heat oil over medium-low heat and sauté shallots with a pinch of salt until tender, about 3 minutes. Add wine, beef stock, and bay leaf and bring to a boil over high heat. Reduce heat to a medium simmer and simmer until mixture has reduced by half, about 15 minutes. Remove the bay leaf and whisk in butter, mustard, and any pan or cutting board drippings. Season with salt and pepper to taste. This can be made a day in advance, stored in the refrigerator, and reheated before serving.
Slice the tenderloin and serve with the red wine shallot reduction on the side.
