If your bread is not already dry and hard, cube it and heat it in the oven at 250°F (120°C) for 20-30 minutes. Another option is to set it out at room temperature for a few hours or overnight.
Preheat the oven to 350°F (180°) and grease your baking dish with vegan butter. Whisk together the flaxseed meal and broth in a large bowl. Set it aside for now.
Heat a small pan to medium and melt the vegan butter in it. Then, add the onions and celery. Sauté them for about 5 minutes or until they're soft.
Add the chestnuts, parsley, sage, thyme, rosemary, garlic, salt, and pepper. Stir as they heat up and become fragrant for 2-3 minutes.
Stir this sautéed veggie mixture into the big broth bowl. Then, gently fold the bread cubes in until everything is combined.
Spread the bread mixture into the baking dish. Bake it uncovered for 45-50 minutes. The stuffing is done when the top is golden-brown and there is no visible liquid in the center.