Butterfly the pork so that it lays flat on a cutting board, fat side down. Season with salt and pepper.
A stuffed pork loin makes for an impressive presentation but really is quite simple to achieve, especially after you learn how to butterfly, stuff, and grill a pork loin like a pro. Quite the impressive trifecta.
Prepare the stuffing according to the package directions.
In a food processor, process the mushrooms, shallots, and garlic until minced. Sauté the mixture in the butter until browned. Deglaze the pan with the Madeira. Combine the mushroom mixture with half of the stuffing (reserve the rest for another use). Chill.
Spread the cooled mushroom mixture on the pork loin, leaving about a half inch at the edges. Roll the pork up. Tie with kitchen twine or silicone cooking twine. Salt and pepper the outside of the pork loin.
Preheat the grill to medium. Put the loin on a roasting rack under a rimmed pan covered in foil.
Grill, lid down, until the internal temperature of the pork - with a digital thermometer inserted in the thickest part of the pork in the center of the roll - reaches 150°F.
Let the roast rest for 20 minutes before removing the twine and slicing.