Spread the brioche on a baking sheet and put it in the oven until lightly toasted, about 10 minutes. Set aside in a large bowl.
In a large skillet over medium-high heat, add the sausage and break it into pieces with a spoon. Cook until browned, then transfer it to the bowl with the brioche. Add the celery to the sausage drippings and cook until softened, about 4 to 5 minutes. Put the sausage into the brioche bowl. Stir in the stock and sage and mix until the brioche absorbs most of the liquid. Season, to taste, with salt and pepper. Transfer the dressing to a 13 by 9-inch baking dish and cook, uncovered for 20 minutes. Cover with foil and bake until golden brown on top, another 20 to 25 minutes.