Form dough into a ball, place in a bowl, and cover with cold water. Cover and let stand 4-8 hours.
Knead the dough and rinse until water runs clear, about 10 minutes. Squeeze dough and press out as much liquid and air as possible. Use a sharp knife or a bench scraper to cut the gluten into bite-sized pieces.
Combine ingredients for broth (or use your preferred vegetable stock) and bring to a boil. Drop gluten pieces into boiling broth and return to a boil. Reduce heat to a simmer. Cook, turning gluten pieces occasionally, until the broth is mostly absorbed and reduced, about 30 minutes. Discard onion and tomato pieces.
To use seitan right away, drain and saute in a little oil. To store, cover with broth and keep refrigerated up to a week, or frozen. Thicken and reduce broth as a gravy if desired.
My favorite ways to eat seitan are on BBQ mock duck pizza, curried mock duck banh mi, in stir fries and fajitas. Leave the packaged 'strips' on the shelf and with just a little effort, make your own unprocessed seitan.