MELT butter in medium saucepan; add onion and cook until translucent. Add water; heat to boiling. Remove pan from heat. Add herb-seasoned stuffing; stir until moistened. Stir in Spinach Soufflé, cooked sausage and 1/4 cup Parmesan cheese.
ARRANGE mushrooms on baking sheet; fill with spinach mixture, mounding slightly. Sprinkle with additional Parmesan cheese.
BAKE in preheated 400°F. oven for 10 to 15 minutes or until cheese and mushrooms are lightly browned.