From America's Test Kitchen
Center-cut pork loin is an especially lean cut, making it difficult to cook without drying out. We wanted to cook our pork loin on the grill but keep it moist using an approach other than traditional brines or sauces. We decided to use a moist, well-seasoned stuffing (and careful cooking) so our grilled pork loin would be juicy and flavorful.
We bought a short and wide roast, more square than cylindrical. This shape only required four straight, short cuts to open to a long, flat sheet that was easy to fill and roll up. The best stuffing required both a deep flavor to counter the pork’s rather bland taste and a texture thick enough to stay put. Poaching apples and cranberries in a blend of apple cider, apple cider vinegar, and spices developed a filling with the dense, chewy consistency we wanted. And this process had an added bonus—we had ample poaching liquid left, which could be reduced to a glaze. We had already decided not to give the loin a preliminary sear, which can create a tough exterior, but found we missed the brown color that searing produces. Rolling the loin in our glaze gave it a beautifully burnished finish.
This recipe is best prepared with a loin that is 7 to 8 inches long and 4 to 5 inches wide. To make cutting the pork easier, freeze it for 30 minutes. If mustard seeds are unavailable, stir an equal amount of whole-grain mustard into the filling after the apples have been processed. The pork loin can be stuffed and tied a day ahead of time, but don't season the exterior until you are ready to grill.