With a small, sharp knife, make 12 (1 1/2-inch deep) slits around the outside of the pork loin. Cut 6 of the garlic cloves in half lengthwise and insert one half clove into each slit. Rub the pork with the Rib Rub, 1 teaspoon of the salt, and 3/4 teaspoon of the pepper. Place the loin in a zip-top plastic bag or glass dish and pour the orange juice and 3 tablespoons of the oil over the pork. Marinate in the refrigerator for 2 hours, turning occasionally.
Remove the pork from the marinade and pat dry. Transfer the marinade to a saucepan, bring to a boil, and reduce to 3 tablespoons, about 6 minutes. Set aside and reserve for the gravy.
Preheat the oven to 425 degrees F.
Combine the beets with 1 tablespoon of the olive oil, 1/4 teaspoon of salt, and 1/4 teaspoon of the pepper in a small bowl.
In a separate, large bowl, combine the onions, parsnips, carrots, potatoes, remaining 4 garlic cloves, and remaining 3 tablespoons olive oil and toss to combine. Season with the remaining 1 teaspoon salt and remaining 3/4 teaspoon pepper.
Arrange mixed vegetables and the beets in the bottom of a small roasting pan. Place the pork loin on top of the vegetables and roast for 20 minutes. Reduce the oven temperature to 325 degrees F and until an instant-read thermometer registers 145 degrees F, 1 hour to 1 hour and 5 minutes.
Transfer the pork to a cutting board to rest for 10 minutes. Tent the pork and the vegetables with aluminum foil to keep warm.
Add the reserved pan juices, shallot, chicken stock, and thyme to the pan with the reduced orange marinade. Bring mixture to a boil and cook until reduced by 1/3 in volume, about 10 minutes.
In a small bowl, whisk together the cornstarch and water. Add to the gravy mixture and bring to a boil. Let the sauce boil for 1 minute, whisking constantly. Remove from the heat and carefully pour into a gravy boat.
Carve the pork, arrange on a large platter, and surround with the root vegetables. Pass the gravy at the table.