Toast the bread slices either in the toaster or on a baking sheet placed under the broiler. Let cool.
Preheat the oven to 375 degrees. Generously butter a 9 x 5-inch loaf pan, then line the bottom with wax paper and butter the paper.
Tear up the toasted bread slices and make crumbs out of them in a food processor. Place in a large bowl.
Process the walnuts until finely ground and mix into the breadcrumbs. Combine the eggs and onions in the processor and process until fine but not liquefied. Stir into the breadcrumbs. Place the green pepper, celery, parsley, tomatoes, and oil in the processor and grind until fine but still with some texture. Stir into the loaf mixture along with the poultry seasoning, salt, and pepper. Mix this all very well until evenly moistened. (The mixture may be prepared to this point and refrigerated up to 8 hours in advance.) Scrape it into the prepared loaf pan and smooth over the top. Cover the loaf with foil.
Bake 1 hour and 20 minutes, or until a knife inserted in the center of the loaf comes out dry. Let sit 5 minutes, then run a knife all along the sides of the loaf to help loosen it. Unmold the loaf onto a platter and remove the wax paper. Let the loaf cool 20 minutes or so before slicing it. It's best to serve the loaf warm and the gravy hot.
Melt the butter in a medium-size saucepan over medium heat. Add the mushrooms and saute until brown, about 7 minutes. Stir in the flour; it will become very pasty. Cook this roux for 2 minutes, stirring constantly. It will stick to the bottom of the pan a little bit; that's okay.
Stir in the stock, wine, soy sauce, and pepper, and bring to a boil. Cook the sauce at a lively simmer for 5 minutes, stirring almost constantly and scraping any crusty bits that adhere to the bottom of the pan. Serve in a sauceboat.
If you don't have a food processor, you can grind everything in a blender, but it will have to be done in many batches so the blender is not overfilled. You can also cook the loaf a few hours in advance, then reheat slices on an oiled baking sheet in a 350-degree oven.
Recipe courtesy of Jeanne Lemlin's VEGETARIAN CLASSICS (HarperCollins, 2001)