Preheat the oven to 275°F. Cut the bread into 1 inch cubes. Spread bread evenly over 1 or 2 baking sheets. Bake until completely dried, about 20 minutes, rotating trays and stirring bread cubes several times during baking. Remove from oven and allow to cool. Increase oven heat to 350°F.
Roughly chop the mushrooms. Roughly chop the brazil nuts. Chop the sage leaves. Halve the roasted chestnuts. Trim the root end and the dark green leaves away from the leek and thinly slice the remainder. Rinse the sliced leek well to remove any grit. Thinly slice the celery. Peel and thinly slice the garlic.
Place a large pot over medium-high heat with 3 tbsp olive oil. Once the oil is hot, add chopped mushrooms and sliced leeks and cook, stirring frequently until all moisture has evaporated, about 6 to 8 minutes. Add the just half of the sage and continue to cook, stirring, until mushrooms are well browned, about 5 minutes longer.
Add the sliced celery, garlic, and remaining sage and cook, stirring frequently, until vegetables are softened, about 10 minutes. Add vegetable stock and Brazil nuts, bring to a boil, and cook until the mixture begins to thicken slightly, about 5 to 7 minutes. Add bread cubes and chestnuts and fold gently until evenly mixed. Season to taste with salt and pepper.
Coat the inside of an 8x8 baking dish with 1 tbsp olive oil and transfer mixture to it. Cover tightly with aluminum foil, and bake until hot throughout, about 20 to 30 minutes. Remove foil and continue baking until golden brown and crisp on top, about 10 minutes longer. Remove from oven, let cool for 5 minutes and serve.