Drain spinach well, pressing between layers of paper towels to remove excess moisture.
Melt 2 tablespoons butter in a skillet over medium heat. Add mushrooms and cook, for 5 minutes. Stir together spinach, mushrooms and cheese; set aside. Roll 1 pastry sheet into a 13 by 11-inch rectangle. Melt remaining 2 tablespoons butter. Brush 1/2 melted butter over pastry. Spread 1/2 spinach mixture over butter. Roll-up, jellyroll fashion, starting with the long side. Repeat procedure with remaining pastry sheet, butter, and spinach mixture. (Wrap rolls in plastic wrap and refrigerate up to 2 days, if desired.) Cut rolls into 1/4-inch thick slices. Place on lightly greased baking sheets. Bake for 15 to 20 minutes, or until golden brown.