Preheat the oven to 350°F. Grease a 9 x 13-inch baking dish with butter or cooking spray.
In a large sauté pan, melt the butter. Add the onions and celery and cook over medium heat, stirring occasionally, for about 8 minutes, or until the vegetables are soft. Add in the garlic and cook for 2 minutes. Scrape all the vegetables in the pan into the bowl with the stuffing cubes.
Take out the chorizo from its casing and, in the same pan, cook the chorizo for 8-10 minutes on medium heat. While cooking, use a metal spatula to break it up into small pieces about 1/4-inch. Add the browned sausage and fat to the bread cubes and vegetables.
Add the chicken broth, egg, rosemary, and sage to the bread cube mixture and mix until the bread is soft and moistened. Transfer the stuffing to the prepared baking dish and bake for 65-75 minutes until golden and crisp on top.