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.
Cover and bake for 45 minutes at 350°F.
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