In a large frying pan or skillet over medium heat, cook sausage until mostly browned, crumbling with a wooden spoon or spatula.
Add onions, parsley, and celery. Cook together for 5 minutes, or until onion is translucent. Remove from heat and let cool slightly.
Pour dry seasoned stuffing into a large bowl or 9x13" baking pan. Add sausage-vegetable mixture, sage, and egg to stuffing in pan. Pour over 2 cups chicken stock and mix well to combine. Add more chicken stock, as desired, to reach desired moistness: 2 cups total for a drier stuffing, 4 cups for a much moister, denser stuffing (our preference).
Stuff in your turkey and bake as directed, or bake in a 9x13" pan at 350º F for 35-40 minutes. Remove from oven and serve alongside carved turkey or chicken.
Recipe Notes
Vegetarian: Omit sausage or replace with plant-based crumbled sausage.
Gluten free: Use gluten free sausage or use gluten free stuffing instead of regular.