Tear bread into 1-inch cubes and place into a large bowl.
Let bread dry in the bowl, uncovered, at least 8 hours.
Melt margarine in a large skillet over medium-high heat. Stir in carrots, celery, and onion. Cook and stir until the onion has softened and turned golden brown, about 10 minutes. Scrape onion mixture into the bowl with the dried bread; season with parsley, sage, and pepper.
Whisk egg replacer, water, and vegetable broth in a small bowl until smooth.
Pour over bread cubes and gently fold until evenly mixed.
Let stand 30 minutes to allow bread to absorb the liquid.
Preheat an oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil and coat with nonstick cooking spray.
Brush seitan with gravy on both sides; arrange overlapping slices on foil in a row, mounding in the center. Press stuffing mixture around seitan to cover. Wet hands lightly and press into a neat oval.
Roll pastry sheets out to 1/16-inch thickness; arrange, overlapping slightly, to make a large rectangle. Seal edges with dabs of cold water. Drape pastry over mound; trim excess away from bottom. Use scraps to cut out decorative shapes (leaves, apples, etc). Attached with dabs of cold water.
Bake until the pastry in deep golden brown, 1 to 1 1/4 hours. Cover the pastry with aluminum foil if it begins to brown too much.
