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

Melt margarine in a skillet over medium heat. Add onion, mushrooms, and celery. Cook and stir until tender, about 12 minutes.

Transfer cooked vegetables to a bowl. Add bread and stir until moistened. Season with salt and pepper.

Cut pork loin in half lengthwise almost all the way through, leaving 1 inch at the bottom uncut. Flatten using a meat mallet and cover with plastic wrap.

Spread stuffing evenly onto flattened pork. Roll up the pork and secure it in three places with kitchen twine.

Place rolled pork loin into a roasting pan. Season with thyme, herbes de Provence, garlic powder, salt, and pepper. Pour about 3 tablespoons water into the pan.

Roast in the preheated oven for about 2.5 to 3 hours or until an instant-read thermometer inserted into the center registers 160°F (70°C).