Preheat the oven to 190°C (Gas Mark 5 / 375°F).
Add the drained cannellini beans to a food processor, along with the garlic, sage, and onion. Blitz to form a chunky paste.
In a large bowl, combine the bean mixture with the dried oregano, vital wheat gluten, vegetable stock, and a good pinch of salt and pepper. Mix well to form a dough (don't be worried if it doesn't look too appetising at this stage!).
Knead the dough for a few minutes - this will help avoid a tough seitan.
Lightly grease a large piece of kitchen foil, and add the seitan dough. Shape to form a rough sausage, then roll tightly in the foil, and scrunch the ends.
Place the foil-wrapped seitan roast on a baking tray (make sure to pick it up from underneath so you don't tear the foil), and roast for around 1 hour 20 minutes, until firm.
Peel back the foil, and slice the seitan roast with a serrated knife. Serve warm.