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

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.

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.

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.

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.