With a small sharp knife, make several ¾-inch deep slits in the meat. Insert garlic slivers into the slits. Brush the meat with 1 Tablespoon olive oil. Set aside.
Preheat oven to 450º F.
In a large skillet, heat unsalted butter over medium-high heat. Brown the meat quickly on all sides. Remove the meat to a rack in a roasting pan. Set skillet aside. Roast meat to the desired doneness, about 20 minutes for medium rare. Remove the tenderloin from the roasting pan to a warm platter. Set aside, covered loosely with foil.
Cognac Mustard Sauce: Melt 1 Tablespoon butter in reserved skillet. Add the shallots and sauté until softened, about 4 minutes. Over high heat, stir in the beef broth; bring to a boil and cook until reduced by half. Add the Cognac and boil 1 minute. Reduce the heat to low. Whisk in the mustard. Cut the remaining butter into 3 pieces and whisk in 1 piece at a time. Stir until the butter is melted. Add the parsley and season with salt and freshly ground pepper. Carve the meat into ½-inch slices. Spoon the sauce over and serve immediately. Yield: 2 servings.