Rinse thawed spinach well and drain as much liquid/moisture as possible by squeezing or pressing onto paper towels. Place in large bowl and set aside.
Melt 2 TBS. butter in large skillet over medium heat. Add mushrooms (I chop mine in a food processor) and saute for about 5 minutes and let cool. Add cooled mushrooms to spinach and mix all together. Roll 1 pastry sheet into 13x11 inch rectangle. Melt remaining 2 TBS. of butter and brush over pastry sheet. Spread 1/2 of the spinach mix over the buttered pastry sheet. Sprinkle 1/2 of grated Gruyere cheese over spinach spread, finishing off by sprinkling 1/2 of the chopped bacon pieces over the cheese. Roll-up pastry (like a jelly-roll) and repeat with remaining pastry sheet, using the rest of the spinach, cheese and bacon. Wrap rolls with plastic wrap for about 15 minutes to make slicing easier). Cut rolls into 1/4 inch thick slices. Place on baking sheets with parchment paper and bake for 30 minutes or until golden brown and bubbling.