Combine oil and vinegar in a cup, rub onto beef. Season generously with salt. Let stand 15 minutes.
Meanwhile, coarsely crush peppercorns and mustard seeds in a blender or food processor or by hand with a mortar and pestle. Roll beef in crushed mixture, pressing it into the surface to coat.
Oil grill to help prevent sticking. Grill beef, on a covered grill, over medium heat 16 to 24 minutes (depending on thickness and your desired doneness). Turn halfway through cooking. Let stand 5 to 10 minutes before slicing. Slice and serve with a few spoonfuls of sauce.
For the Dijon-Cream Sauce, bring beef broth and whipping cream to a boil in a saucepan. Boil gently until reduced to about 1 cup; sauce will be thick enough to coat a spoon. Remove from heat; stir in butter, a little at a time, until all the butter is melted. Stir in mustard and vinegar, adjusting amounts to taste. Sprinkle with peppercorns and mustard seeds. Makes about 1 cup.