Heat 1/2 cup water in a large pot or skillet. Add the onion and cook 5 minutes. Add the sliced mushrooms & celery and cook over medium heat, stirring occasionally, until the mushrooms begin to brown, about 5 minutes. Preheat oven to 350F. Stir in bread & spices, salt & pepper. Lower the heat & continue cooking for 3 minutes, then stir in the water or stock, a little at a time, until the dressing obtains the desired moistness. Spread in an oil-sprayed baking dish, cover & bake for 20 minutes. Remove the cover and bake 10 more minutes.