Preheat oven to 200 degrees. Arrange bread on a parchment-lined baking sheet. Bake until dried out, about 8 minutes. Let cool.
Raise oven temperature to 350 degrees. Heat oil in a large nonstick skillet over medium-high heat. Brown sausage, breaking up meat with a wooden spatula, for 4 minutes. Stir in celery, garlic, and onion. Reduce heat to medium. Cook, covered, stirring occasionally, until vegetables are soft, 10 to 12 minutes. Let cool slightly.
Combine bread, sausage-vegetable mixture, stock, egg, parsley, thyme, and sage in a large bowl. Transfer stuffing to an 11-inch oval baking dish. Cover with parchment-lined foil. Bake for 45 minutes. Turn on broiler. Uncover stuffing, and broil 9 to 10 inches from heat source until top is golden brown and crunchy, about 1 minute. Let stand for 5 minutes before serving.