Preheat the grill to medium heat. Using a paring knife, cut a gulley down the middle of the pork tenderloin end to end. Make sure NOT to cut all the way through. Open the pork loin and lay it flat. Make additional small cuts to open it wider if needed.
Slather the pork tenderloin with dijon mustard on both sides. Sprinkle with salt and pepper on both sides.
Grill for 5 minutes per side. Remove from the grill and tent with foil for 5-10 minutes before slicing.