Preheat the oven to 375 F.
Add the sausage to a large skillet over medium heat and cook for 8-10 minutes, breaking it up as it cooks, until browned, then remove the sausage from the skillet.
Add the onion, celery, and garlic to the skillet and cook for 5-7 minutes, until softened, then remove from heat.
Remove the seasoning pack from the stuffing mix, then add the stuffing croutons, sausage, cooked veggies, herbs, salt, egg, and stock to an 8x8 inch pan or other 3-quart baking dish and mix it all together. Press the stuffing into an even layer in the pan, then drizzle the top with butter.
Bake for 35-40 minutes, until the top of the stuffing is browned and crisp. Let cool slightly, then enjoy.