Place pork in a shallow dish or a zip-top plastic freezer bag; pour mustard mixture over pork. Cover or seal, and chill 2 hours, turning occasionally.
Remove pork from marinade, discarding marinade.
Grill, covered with grill lid, over medium-high heat (350° to 400°) 10 to 12 minutes on each side or until a meat thermometer inserted into thickest portion registers 155°. Remove from grill; let stand until thermometer registers 160°. Let stand 5 more minutes before slicing. Garnish, if desired.