Preheat oven to 350°F.

Add all the ingredients except for the fennel seed and vital wheat gluten into the bowl of a food processor. Process until well-combined and smooth, scraping down the bowl with a spatula as needed to ensure everything is evenly blended.

Add in fennel seed and vital wheat gluten. Process until all the gluten is absorbed (i.e. there are no dry patches in the mix). If the dough seems very wet or sticky, add an additional 2 tablespoons vital wheat gluten.

Run the food processor for 3-4 minutes to knead the dough for you and rapidly develop the strength of the gluten. The dough will initially break into lots of different crumbles, but don't worry; over time they will come back together and form one large ball. This is how you'll know it's sufficiently kneaded.

Split dough into two equal halves and shape into logs. Vary the thickness depending on how big you want your slices of pepperoni to be. Then wrap each log in a layer of parchment and a layer of aluminum foil, twisting the ends tightly like a Tootsie roll to seal.

Place wrapped seitan logs in a baking dish, seam-side up. Pour in enough water to cover the logs halfway, then bake for 50 minutes minutes. At 25 minutes, flip the logs over using tongs and bake for another 25 minutes.

Remove from the baking dish and allow them to cool entirely in the wrappers. The pepperoni will firm up considerably after 4+ hours in the fridge, so it's best to make it in advance so that it's easy to slice.