Prepare the Seitan "Beef": In a large bowl, mix together the vital wheat gluten, chickpea flour, nutritional yeast, paprika, cumin, and salt. In a separate bowl, whisk together the vegetable broth, minced garlic, and olive oil. Gradually add the wet ingredients to the dry, mixing until a dough forms. Knead for about 3–5 minutes until firm. Shape into a thick loaf.
Simmer the "Beef": In a large pot, bring 8 cups of water or vegetable broth to a gentle simmer. Add salt, brown sugar, mustard seeds, and 1 tablespoon of mixed pickling spices. Carefully place the seitan loaf into the simmering broth and let it cook for 1.5 to 2 hours, turning occasionally. Keep the heat low to prevent boiling.
Cook the Vegetables: In another pot, bring fresh water or vegetable broth to a boil. Add the remaining 1 tablespoon of pickling spices. Add potatoes and carrots and cook for about 15–20 minutes. Then, add the cabbage and continue cooking for another 10 minutes, or until all vegetables are tender.
Slice and Serve: Once the seitan is done, let it rest for 10 minutes before slicing thinly against the grain. Arrange the sliced "beef" and cooked vegetables on a platter, ladle some of the broth over the top, and serve warm