Roast Beef Tenderloin with Cognac Butter

Using a tenderloin, a thinner cut of steak, you can enjoy a flavorful buttery filling. You will use only half of the brandy oil, but prepare it completely - you will not be able to halve the amount either. Use the remaining butter for a week for pasta or fish, or freeze for one month; bring to room temperature before serving.
To make the brandy butter, melt 1 1/2 teaspoons of butter in a small nonstick skillet over medium-low heat. Add the shallots; cook for 2 minutes or until tender, stirring the shallots occasionally. Stir gently in the brandy; cook for 1 minute or until the liquid is reduced by about a third. Remove from heat; allow to cool.Put 6 1/2 tablespoons butter in a small bowl; add the chilled brandy mixture, 1 tablespoon thyme leaves, and 1/8 teaspoon black pepper. Cover and refrigerate for 10 minutes. Divide the butter mixture in half. Spread each half of the butter mixture on a piece of plastic wrap; shape each portion into a 4-inch log. Wrap each stick of butter in plastic wrap; refrigerate or freeze 1 stick for another use.Preheat the oven to 425 degrees.To make the tenderloin, combine the mustard and the following 4 ingredients (with 1/2 teaspoon pepper); whisk to combine. Spread the mustard mixture on all sides of the tenderloin; sprinkle with 2 tablespoons of thyme. Place the tenderloin in a shallow baking dish greased with cooking spray. Bake at 425 ° for 38 minutes or until a thermometer placed in the center of the tenderloin reads 135 ° or until the desired degree of roasting. Let stand for 10 minutes.Cut the tenderloin crosswise into 16 slices. Place 2 slices on each of 8 plates. Cut 1 stick of butter into 8 slices; garnish each serving with 1 slice of butter.