Toast the bread cubes for 8-10 minutes to dry them out. Alternatively, you can use day old bread that is already dried out.
Increase oven temperature to 350F.
Heat oil in a sauté pan. Add the sausage, apples, onions and celery, and sauté for 5-7 minutes, until everything is heated through and the veggies are tender*. Allow to cool a few minutes before continuing.
Place all the ingredients in a large bowl and toss to thoroughly combine.
Spoon the stuffing mixture into greased muffin tins, filling all the way to the top. Pat the mixture down to pack it well.
Bake for 45 minutes.
*Note: If you’re using sausage that is not fully cooked, sauté it for 4 minutes before adding the oil and the remaining ingredients.