Dry bread crumbs out. Two days before baking Dressing, slice bread and let dry on a wire cooling rack for 24 hours. The next day, cube the sliced bread with a knife, place in a large bowl and allow to continue to dry for another 24 hours. Toss them throughout the day to make sure they are all getting enough air to dry. *See note for quicker drying process.
In a large pan, melt the butter on the stove top on medium heat.
Add onion and cook until tender.
Remove from heat and pour the butter and onions over the bread crumbs. Stir until all the bread crumbs are covered with the melted butter.
In a separate bowl, combine the salt, pepper, sage, thyme and poultry seasoning. Sprinkle the mixture over the bread crumbs and toss until all the bread crumbs are coated.
Add the chopped celery and toss until just combined.
Add the chicken broth, a little at a time. Stir the bread crumbs as you go. Add just enough chicken broth to completely moisten the bread crumbs. Too much broth will make mushy Dressing.
Spread into a 9x13 casserole dish. Cover with a lid or foil.
Bake in preheated 350°F oven for 20 minutes. Remove the lid or foil and bake an additional 10-15 minutes or until slightly golden.