Meanwhile, in a large, deep skillet, melt the 1 stick of butter. Pour half of the butter into a small bowl and reserve. Add the celery, carrots and onion to the skillet and cook over moderately high heat, stirring occasionally, until softened and just beginning to brown, about 8 minutes. Scrape the vegetables into a large bowl. Add the sausage to the skillet in lumps and cook over moderately high heat, breaking it up with a spoon, until lightly browned and cooked through, about 6 minutes. Return the vegetables to the skillet, add the sage and thyme and cook for 1 minute. Add 1 cup of the stock and cook, scraping up any bits stuck to the pan, until nearly evaporated, about 5 minutes.