With a small sharp knife, trim surface fat and the thin silvery membrane from tenderloins; rinse meat and pat dry.
Sprinkle evenly with pepper.
Put mustard seed in a pan longer than the tenderloins; roll meat in seeds until heavily coated.
At this point the tenderloins can be covered and chilled as long as overnight.
Place seed-coated meat on a heated grill.
Cook meat about 15 to 20 minutes, or until only slightly pink in the center of the thickest part (cut to test), or until meat thermometer registers 155 degrees.
Turn often to keep browning uniform.
Transfer to a carving board and brush seeds with melted butter to make shiny.
Slice meat across the grain and serve with mustard and salt to taste.