Cut bread into cubes totaling 10 cups and spread it on a cookie sheet. Let it sit overnight to dry out or pop it in a 300° F oven for about 20 minutes until the bread is dry and crispy. (You want it to just dry out, not brown!)
In a large saucepan, saute a finely diced onion in 1/4 cup of butter (or olive oil) over low heat until the onion becomes translucent. (This will take about 5-7 minutes.)
Add the chopped celery, carrot, and mushroom to the onions and sprinkle with poultry seasoning and salt. Stir well and saute the veggies for about 5 more minutes until they are soft.
Add 2 cups of broth to the pot and bring to a boil.
Turn off the heat and add the dry bread to the pot and give it a quick stir.
Cover with a lid and let it sit for 10 minutes before fluffing.
Notes
Be sure to cook the veggies as tender as you want them before adding the broth and stuffing since they won't cook much after that.
Don't simmer the broth, the liquid will evaporate and you won't have the correct broth to breadcrumb ratio anymore.