Place all ingredients in a medium sauce pan. Bring to a boil over high heat, stirring to break up the cranberry sauce. Turn down to a simmer and cook until sauce is thick enough to coat a spoon, about 8-10 minutes. Taste for salt and pepper.

For the beef:
Preheat oven to 425°F. Rub meat all over with the oil. Combine the salt, pepper and mustard seed and sprinkle mixture all over the meat. (You might have to press mixture on roast if mixture doesn't adhere.)

Set tenderloin on a wire rack on a shallow roasting pan. Roast until a thermometer inserted into the thickest part registers about 135° for medium-rare to medium. Let stand at room temperature about 20-25 minutes before slicing.

Cut meat into 1/4 to 1/2 inch thick slices. Arrange overlapping slices on a serving tray and spread a 2-inch wide "ribbon " of cranberry mustard sauce down the center. Pass the remaining sauce.