Heat oven to 450 degrees F.
Rub tenderloin with oil. Season with salt and pepper. Cover surface of tenderloin with garlic, pressing to adhere. Place meat on rack in roasting pan.
Roast in 450 degree F oven for 40 minutes or until internal temperature registers 140 degrees F on instant-read thermometer for medium-rare. Let meat stand in warm place 10 minutes before slicing for serving (internal temperature will rise to 145 degrees F).
While meat is roasting, in medium-size saucepan, heat butter over medium heat. Add shallot and garlic; cook over medium-low heat 5 minutes, until softened. Add thyme, peppercorns, port and all but 4 tablespoons broth. Bring to a boil. Lower heat; simmer 15 minutes.
In bowl, stir together reserved 4 tablespoons beef broth and flour.
Strain broth mixture with shallots through sieve placed over a bowl; discard solids. Pour back into saucepan. Stir in broth-flour mixture. Bring to a simmer; cook, whisking occasionally, 2 minutes or until slightly thickened.
Serve sliced meat with port sauce on the side, and roasted potatoes, if desired. Makes 16 servings.