Step 1

Sauté the onion and celery in a non-stick skillet until onion is becoming translucent. Add the mushrooms, thyme, sage, and a generous grating of black pepper and cover. Cook until mushrooms exude their juices, about 3 minutes. Add the remaining ingredients along with enough water to moisten the stuffing but not make it soaking wet. Remove from heat and keep covered.

Step 2

In a mixing bowl, combine the dry ingredients (vital wheat gluten through chia seeds). Place the 1 1/2 cups of broth, white beans, soy sauce, and garlic in blender and process until liquefied. Make a well in the center of the dry ingredients, add the bean mixture, and stir until gluten is completely moistened. Drizzle the tahini over the top and knead it into the dough. Keep kneading until dough holds together in a ball. Set aside while you make the broth.

Step 3

Heat all ingredients until hot but not boiling. A microwave works well for this.

Step 4

Preheat oven to Lightly oil an oval or rectangular baking dish, 11-13 inches long and 6-8 inches wide. (Your seitan will expand to fit it, so try not to use a very wide dish.)

Step 5

Line your work surface with plastic wrap, parchment paper, or waxed paper. Place the dough in the center, cover it with plastic wrap, and roll out the seitan, making sure that it is the same thickness in all places, until it's about 9x13 (an inch or so either way doesn't matter, but make sure it's not longer than your pan). Spread the stuffing evenly, leaving a 1-inch margin on all sides.

Step 6

Lift up the plastic wrap on one of the long edges and roll the seitan up like a jelly roll. (Alternatively, arrange the stuffing in a horizontal line across the middle of the seitan and bring one long edge up and over it to the other side.) Pinch the ends sealed first and then pinch well to seal the long seam. Take care to make sure that the edges are completely sealed and no gaps or stuffing shows.

Step 7

Lift the seitan roll carefully and place seam-side down in the prepared casserole dish. Pour the baking broth over it, add rosemary, and cover tightly. If the dish doesn't have a cover, use aluminum foil to cover tightly. (Did I mention "tightly?" Tightly! I enclosed even the bottom of the dish in foil.)

Step 8

Bake for 25 minutes. Remove from oven, baste with broth, recover tightly, and bake for another 25 minutes. Baste again and return to oven uncovered for about 30 minutes. Baste 2 or 3 times as it's cooking. Seitan is done when top seems firm and brown and the broth has evaporated. You can test it by cutting a small slit in the middle; if it is doughy rather than firm, return to the oven.

Step 9

Remove from the oven and let cool for 5-10 minutes. Transfer carefully to a cutting board or serving platter and cut into 1/2-inch slices.