If baking separately as a dressing, preheat the oven to 350°F and butter an 8-inch square baking dish.
Cook in a medium skillet over medium heat, breaking up the meat with a spoon, until browned:
Using a slotted spoon, transfer the meat to a plate lined with paper towels to drain. Pour off all but 2 tablespoons of fat from the skillet. Add and cook until softened:
Transfer the onion mixture to a bowl and add the sausage along with:
If baking as a dressing, also add:
Stir to combine. Moisten with:
Spoon the stuffing into the meat or vegetable being stuffed. For a dressing, transfer to the prepared baking dish and bake until browned, about 30 minutes. If using to stuff poultry, cook until it reaches an internal temperature of 165°F.