Heat large saute pan over high heat. Add the sausage and cook until golden brown. Remove the sausage with a slotted spoon to a plate lined with paper towels. Add butter to the rendered fat in the pan, and then add the onions, celery and carrots and cook until soft. Add the garlic and cook for 1 minute. Remove the mixture from the heat and fold in the sage. Scrape the mixture into a large bowl and let cool slightly.
Preheat the oven to 425 degrees F.
When the vegetable mixture has cooled slightly, add the sausage to the bowl. Add the egg and bread and mix to coat. Add the chicken stock until the mixture is moist. Season with salt and pepper, to taste.
Butter a large baking dish and scrape the mixture into the dish. Bake for 25 to 30 minutes or until golden brown.