A day or two in advance, let the bread cubes dry in a bowl or the slow cooker uncovered before making the stuffing.
Preheat a large skillet to medium-high. Add the sausage and cook for 15 - 20 minutes, breaking up with a spoon, until the sausage is crumbly, browned, and cooked through.
Add the onion and celery to the sausage with the salt, pepper, and poultry seasoning and cook for 2-3 minutes more to soften the veggies just a bit.
Add the bread cubes, apple, and sausage mixture to the slow cooker. In a bowl, whisk together the melted butter, eggs, and stock. Pour the butter mixture over all the ingredients, and stir to combine.
Press the stuffing into the slow cooker with the back of a spoon, then cover and cook on low for 3-4 hours until the edges are slightly crusty and the center of the stuffing is set.
Taste the stuffing and add another sprinkle of salt right before serving if you'd like. Serve hot straight from the slow cooker.