Step 1

Preheat oven to 300 degrees F (150 degrees C).

Step 2

Heat margarine in a skillet over medium heat; cook and stir onion, mushrooms, and celery in the hot margarine until vegetables are tender, about 12 minutes. Transfer vegetables to a bowl. Stir bread into vegetables until moistened; season with salt and black pepper.

Step 3

Cut the pork loin almost in half lengthwise, cutting to about 1 inch from the bottom; open pork loin like a book. Cover with plastic wrap and lightly pound with a meat mallet to flatten the meat. Spoon stuffing onto the pork loin, roll the meat over the stuffing, and tie pork loin together in three places using kitchen twine.

Step 4

Place tied loin into a roasting pan and pour about 3 tablespoons water around the roast. Season roast with salt, black pepper, thyme, herbes de Provence, and garlic powder.

Step 5

Roast stuffed pork loin until an instant-read meat thermometer inserted into the center of the stuffing reads 160 degrees F (70 degrees C), 2 1/2 to 3 hours. Rest meat for 15 minutes before untying and slicing.