Melt the butter in a heavy pan over medium heat. Sauté the onion and celery for 10 minutes, or until soft. Add the sausage, mushrooms, spinach and thyme and cook for 2 minutes, until the spinach is wilted.
In a large bowl, mix together the bread cubes, sausage-vegetable mixture and Parmesan. Moisten with stock until its moist enough to hold together when molded on a spoon.
Taste for salt and pepper. Place stuffing in a buttered casserole dish.