Preheat the grill for indirect medium heat (~375°F).
In a small bowl, mix the panko, cranberries, olive oil, and rosemary. Season with salt and pepper.
Season the pork with salt and pepper. Cut a slit about 2 inches deep the length of the tenderloin. You want to cut a slit deep enough to hold the stuffing, but not all the way through the meat.
Spoon the stuffing into the slit, then tie the pork at 2 to 3 inch intervals using butcher's twine.
Brown the pork over direct heat on all sides except the side filled with the stuffing, about 2 minutes per side.
Transfer to indirect heat and cook until the pork and stuffing reaches an internal temperature of 145°F, 23 to 25 minutes longer. Transfer to a cutting board and let it rest for 5 to 10 minutes.
While the pork is resting, pour the orange juice into a small cast iron skillet. Set it over direct heat on the grill (or on a side burner or stovetop). Heat to a simmer, then whisk in the Dijon mustard until smooth.
Remove from the heat and whisk in the butter, one cube at a time, letting each one melt before adding the next one. Whisk until the sauce is smooth and emulsified. Season to taste with salt and pepper.
Slice the pork into 1-inch thick medallions and serve with the sauce.