Two hours before cooking, remove the pork loin from the over. Rinse the pork loin and pat dry with paper towels. Place the pork loin on a baking sheet or roasting rack on top of an oven-proof roasting pan or sheet pan.
Combine the olive oil, rosemary, salt, lemon zest, and mince garlic in a bowl and stir to combine. Spread the herbed olive oil mixture over the entire pork loin, ensuring it covers the sides and bottom side of the loin, as well as allowing it to drip in between the score marks of the pork on top. Allow the pork to sit at the room temperature for 1-2 hours before cooking.
Preheat a grill or over for cooking at 325 °F. Place the entire pan and roasting rack holding the pork loin on the grill or in the oven. Allow to cook for 2-3 hours, or until the internal temperature of the pork loin reaches 155 °F. There is no turning necessary.
Remove the pork from the grill or oven and place on a cutting board. Tent loosely with the foil and allow the pork to rest for 15-20 mins, allowing the juices to reabsorb into the meat and for the pork to finish cooking. It should increase in temperature by 5-10 degrees.
Pour the juices from the pan into a fat separator and allow to cool. Discard the fat. Pour the remaining juices into a small saucepan and warm. If the juices are too salty, add a small amount of demi-glace or low-salt stock to the pan.
When ready to serve, slice the pork loin into 1″ slices, ensuring some of the crispy skin remains on each slice. Drizzle with the pan juices.