1. In the bowl of a food processor, combine the onion and celery and pulse until finely chopped.
2. Heat 2 tablespoons of the oil in a nonstick skillet set over medium heat. Add the onion mixture, salt, and pepper and cook, stirring, until golden brown, about 7 minutes. Transfer to a medium bowl and set aside.
3. Tear the bread into 2-inch pieces, put them in the food processor, and pulse until most of the mixture is in fine crumbs. Transfer to the bowl of onion mixture.
4. Put the mushrooms and parsley in the food processor (no need to wipe it out) and pulse until finely chopped. Add the mushroom mixture to the onion mixture.
5. In a small bowl, combine the tapioca starch and tamari. Sprinkle it over the onion mixture. Using your hands, work the onion mixture until it begins to stick together and is almost pasty. Let stand for 10 minutes before forming into 4 patties about 4 inches across.
6. Heat the remaining 2 tablespoons oil in a large nonstick skillet set over medium-low heat. Cook the patties until golden brown, about 4 minutes. Flip the patties and cook until the second side is golden brown, 3 to 4 minutes.
7. Heat or lightly toast your burger buns in the oven or toaster (350 degrees for 5 minutes). Drizzle and spread your favorite condiment(s) on one half of your toasted bun and then carefully place your veggie burger patty on top, using a spatula. Layer fresh lettuce leaves, tomato and onion slices and top with your other bun half.
Yield/Servings: 4 burgers