Place the pork loin in a plastic food storage bag. (Cut it in half if necessary to make it fit.) Add the marinade ingredients. Push most of the air out of the bag and seal it. Marinate in the refrigerator for three to five hours.
About a half-hour before serving time, remove the pork loin from the marinade, letting most of the liquid drip off.
Heat the butter in a heavy skillet over medium heat until it bubbles. Brown the pork loin on all sides. (Don't be concerned with cooking the interior just yet.)
When browned, turn the heat off and move the pork loins to a roasting pan. Roast in a 375 degrees until the internal temperature reaches 160 degrees on a meat thermometer pushed into the center. (About 45 minutes.)
Pour the marinade ingredients into the pan in which you browned the pork, over medium heat. Whisk to dissolve the juices and browned bits in the pan. Reduce the marinade, stirring often, until it thickens enough to coat the back of a spoon. Taste the sauce and adjust seasonings with salt and pepper.
When the pork loin is cooked, allow it to rest for five minutes. Cut into slices about 1/3 inch thick. (If using tenderloin, cut it on the bias.) Divide the slices on plates and nap with the sauce. Garnish with orange or kumquat slices.
