If using fresh bread instead of stale, preheat the oven to 250°F (120°C). Cut the loaf of bread into ½ inch (1.25 cm) cubes. Spread the bread cubes on a baking tray and place them in the oven to dry out for 30-35 minutes. Remove and allow them to cool. Adjust the oven temperature to 350°F (175°C).
Rinse the leeks thoroughly to remove surface dirt, then slice them and immerse in a bowl of warm water. Gently massage the leeks to release any hidden grit, then drain them in a colander or wire mesh strainer, and rinse under cold running water until completely clean.
Place a large frying pan over medium-high heat and add the vegan butter. Once melted, add the onion and leeks. Sauté, stirring often, for 6-7 minutes until the onion and leeks start to brown lightly.
Add the celery, garlic, sage, parsley, and thyme to the pan. Continue sautéing for 2-3 minutes just until the celery has softened slightly. Remove the pan from the heat.
Heat the vegetable stock in a saucepan over high heat until it reaches a boil.
In a large mixing bowl, toss the cubes of bread with the cornstarch. Add the cooked herb and vegetable mixture, pour in the boiling stock and season with tamari and pepper to taste.
Transfer the bread mixture to an oven-proof dish. Gently stir in optional sliced chestnuts, walnuts, or dried cranberries, then cover the dish with foil. Roast in the oven at 350°F (175°C) for 18 minutes, then remove the foil and continue roasting for an additional 10 minutes. Remove the large baking dish from the oven and either serve directly from it or transfer the stuffing into an attractive serving dish.
Garnish with optional additional pats of vegan butter, fresh parsley leaves, sage, or thyme sprigs.