From America's Test Kitchen Season 1: Perfect Pork
Although it has a little more fat than pork tenderloin, a center loin pork roast is still quite lean and requires special handling to roast without drying out. We sought the best way to roast this cut so that the juices would remain inside the meat, not wind up on the carving board.
It turns out that a two-step roasting process is the key to juicy pork loin. After poking slivers of garlic into the meat and rubbing the surface with a mixture of thyme, cloves, salt, and pepper for extra flavor, we refrigerated the roast overnight. The next day we cranked up the oven to 475 degrees and added the pork directly from the fridge, leaving it for just half an hour before removing it. After we rested the roast, we returned it to the oven, this time at a lower temperature, to finish cooking. The texture of the meat was remarkably tender, and it had lost very little juice. It turns out the reason this method works is that during the rest, the middle of the roast heats by conduction from the heat absorbed by the outside of the roast. When the meat goes back into the oven, the center cooks through but the outside doesn’t overcook. A mustard-shallot sauce provides additional moisture and flavor, although the roast is so juicy and flavorful on its own, it can be omitted.
Serves 4 to 6
A thin, flat pork loin roast will overcook. To avoid this overcooking, tie the roast yourself, or have the butcher perform the task for you.