Place the slices of day-old bread on large baking sheets and toast under your oven broiler (turning to toast both sides until golden brown and very crispy. Remove from the oven and set aside to cool.
n a large skillet, drizzle some olive oil and, on medium heat, sauté the chopped onion for about a minute then add a good dose of white wine and let this sauté until the wine has evaporated and the onions are tender and sweet. Season with salt and pepper to taste; remove from skillet and set aside.
In this same skillet on medium-high heat, brown the loose sausage meat until brown and cooked through. Let the meat carmelize a bit with those browned edges; this adds tons of flavor to the final dish.
Take the previously sautéed spinach (remove garlic clove) and chop the leaves into bite-size pieces. Using your hands, break the toasted bread up into large chunks and place in a large deep bowl.
Add the sautéed onion and spinach (add the spinach liquid too) and mix gently to blend with a large wooden spoon or spatula.
Chop the chestnuts into quarters and add them to the bread mixture. Do the same with the cooked sausage meat once cooled and mix well, careful to not break up the bread pieces.
Add the chopped raw carrots and celery and gently mix together. Season this mixture with some salt and pepper (do not over salt here as the sausage is already a bit salty.)
Begin to add the stock slowly to saturate the bread pieces, do not add too much stock as it will end up a big, soggy mess once cooked.
When the bread has absorbed the stock; add more if needed and put this finished mixture in a large greased oven roast pan with high sides.
Cover the pan loosely with tin foil and place into a preheated 400 degree fahrenheit oven on the middle rack.
Roast this mixture for about 20-25 minutes and then remove the foil and continue cooking until the top gets golden toasty and crisp on the edges.
Remove the pan from the oven and set aside to cool a bit; serve immediately.