Preheat oven to 350 degrees. Combine first 5 ingredients in a small bowl. Pat pork loin dry with paper towels.Rub spice mixture all over pork and place pork fat side up in a roasting pan.
Roast for 45 minutes.Meanwhile make glaze. Combine orange juice, brown sugar, Dijon mustard, soy sauce, and red pepper flakes in a small saucepan. Stir together water and cornstarch in a small bowl. Add to saucepan.
Bring mixture to a simmer, whisking constantly. Once thickened, remove from heat. If it gets too thick, add more water.
Once pork has roasted for 45 minutes, baste it with glaze. Continue roasting until it reaches an internal temperature of 145 degrees. This should take about 15 to 30 additional minutes.
When pork is fully cooked, brush it with more glaze and let rest 10 minutes before slicing. Serve it with any remaining glaze.