Preheat oven to 350 degrees Fahrenheit
Add one tablespoon of olive oil to a large skillet and brown the sausage, breaking it apart until there is no more pink and it is cooked through.
Transfer sausage to a paper towel lined bowl and set aside.
In the same skillet in which the sausage was cooked, melt 12 tablespoons of butter and add the diced onions, celery and mushrooms. Stir frequently until onions are translucent and mushrooms have reduced and are cooked through.
In a very large mixing bowl, combine breadcrumbs, sausage, the onion/celery/mushroom mixture, poultry seasoning, sage, marjoram, rosemary, salt, pepper and eggs and mix together until just combined.
Gently pour chicken broth over the mixture and mix again.
Transfer to a 9 X 13 baking dish and cover lightly with foil.
After 15 minutes, remove the foil and continue baking for 30 more minutes
Remove from the oven and garnish with chopped fresh parsley