Gather all ingredients. Preheat the oven to 300 degrees F (150 degrees C).

Make the stuffing: Melt margarine in a skillet over medium heat. Add onion, mushrooms, and celery; cook and stir until tender, about 12 minutes.

Transfer vegetables to a bowl. Add bread, stir until moistened, and season with salt and pepper.

Make the pork: Cut pork loin in half lengthwise almost all the way through, starting at the top and stopping at the last inch at the bottom. Leave the bottom inch uncut and open pork like a book.

Cover with plastic wrap and lightly pound with a meat mallet to flatten. Remove and discard plastic wrap.

Spoon stuffing onto flattened pork loin.

Roll meat over stuffing and secure in three places with kitchen twine.

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

Roast in the preheated oven 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. Remove from the oven and let rest for 15 minutes before untying and slicing.