Make dough: Combine dry ingredients in a bowl and wet ingredients in another bowl. Add wet ingredients to the dry, mixing with a fork until blended completely. Transfer dough to a clean, flat surface and knead by hand 10 times. Let rest 3 minutes. Knead 10 to 15 times
more; let rest 15 minutes more.
Meanwhile, in a pot, bring broth ingredients to a boil. Reduce to a simmer and cover.
Roll dough into a thick log (about 3 inches wide by 8 inches long). Cut crosswise into eight 1/2-inch-thick disks and flatten with hands to about 1/4-inch thickness. Add disks to broth, cover, and simmer until the middle of the dough is as firm as the edges, 1 to 1 1/2 hours. (If the disks become exposed during cooking, add just enough water to cover. Be sure the broth does not boil; this produces spongy seitan.) Remove from heat and let cool, uncovered, in broth.
Once cool, seitan can be marinated, refrigerated in the broth for up to a week, or frozen individually in parchment for up to a month.