Preheat oven to 300 degrees. Tear bread slices up into bite size pieces. Place onto two cookie sheets and toast in the oven for about 30-40 minutes. Check and turn bread every 10 minutes. You only want it slightly brown, but dry through out. Cool completely and transfer to a extra large bowl.
In a large pot add chicken stock, diced celery, poultry seasoning and salt. Bring to a boil and cook for 20 minutes.
Meanwhile, in a large pan break up sausage and brown over medium high heat. Drain onto a paper towel and add to bread. Using the same pan, melt butter over medium high heat and add in diced onions. Saute until translucent and slightly browned. Pour onions and butter over bread and toss together.
Slowly ladle in seasoned chicken stock and celery over the bread mixture and stir to combine. Don't add in all the stock at once because you might not need it all. Just add enough till the bread is completely moist, but not drenched.
Preheat oven to 350 degrees. Place stuffing into a 9x13 pan and bake for 40 minutes.
You can make this ahead. Once you've placed it in a 9x13, cover well with plastic wrap and place in the refrigerator. When you are ready to bake it, let it sit on the counter for 30 minutes and bake it for 40-50 minutes at 350 degrees.