In bowl mix the salt, pepper, garlic powder, mustard and olive oil. Spread the mustard mixture over all of the sides of the pork loin. Place the pork loin fat side up in the oven. Bake for 1 hour and 20 min or until the internal temperature reaches 145F/63C. Flip the pork halfway through cooking to brown the other side.
While the pork is cooking heat a heavy bottomed pan on medium high heat and add the apple cider, brown sugar, dijon mustard, corn starch and apple cider vinegar. Whisk to make sure all of the cornstarch has disappeared. The sauce will boil and thicken continue to whisk until it can coat the back of a spoon and a line drawn through it stays.
Once the pork has been removed from the oven allow the pork to rest at least 10 minutes before slicing. Drizzle the glaze over the sliced pork.