Trim any excess fat from the pork loin, leaving a thin-layer of fat over the top.
Evenly rub the entire pork loin with the Dijon mustard followed by the onion, salt, pepper, and garlic. Make sure to really rub in the spices, and then sprinkle the
rosemary evenly all over the top.
Place the pork in roasting pan lined with a rack and roast for 20 minutes.
Reduce the temperature to 300 degrees F, and continue roasting until a meat thermometer inserted in the thickest part reads 145 degrees F, about 1 hour more. Transfer the pork to a cutting board, cover loosely with foil, and let rest for 10 minutes. Slice and pork and arrange on a platter garnished with rosemary sprigs.