If using fresh bread instead of stale, preheat the oven to 130°C. Spread the bread cubes in a single layer on a baking tray and place in the oven for 20 minutes to dry out. Once crisp, remove from the oven and allow them to cool completely. Adjust the oven temperature to 170°C.
In a large frying pan over medium-low heat, melt the vegan butter. Once fully melted, add the chopped onion and sage. Cook gently for about 15 minutes, stirring regularly, until the onions turn golden and fragrant.
Stir in the celery, minced garlic, parsley, and rosemary. Continue cooking for a few more minutes, allowing the celery to soften and the herbs to release their aroma. Remove from heat and set aside.
In a large mixing bowl, toss the bread cubes with the cornstarch until evenly coated. Add the sautéed onion and herb mixture, stirring well to combine. Season with salt and black pepper to taste.
For stuffing balls, pour the hot vegetable stock over the mixture, stirring until everything is well moistened. Cover the bowl with a clean tea towel and let the mixture absorb the liquid for 5 minutes. Using your hands, shape the mixture into 10–12 evenly sized balls and arrange them on a lined baking tray. Bake for 20 minutes at 170°C until golden and crisp on the outside.
For traditional scoopable stuffing, transfer the prepared mixture into an ovenproof dish. Evenly pour the hot vegetable stock over the mixture, ensuring it is fully absorbed. Bake in the preheated oven for 20 minutes, until the top is lightly browned and slightly crisp.
Remove from the oven and serve immediately as a flavorful, herb-infused side dish.