Poach chicken in wine for 8 minutes; remove from cooking liquid (discard liquid); allow chicken to cool completely and cut into small dice; put in a large bowl and set aside.
Rinse spinach leaves, shake most of water off leaves and place in large saucepan over high heat; cover pan and steam spinach for 3-4 minutes, or until it has just wilted; tip into a colander and run cold water over spinach to stop cooking; leave in colander to drain, pressing down with palm of hand and compacting into a bunch; press out as much liquid as possible; chop spinach up quite fine and add to chicken.
Over medium-high heat, saute onion in olive oil in large frying pan, stirring occasionally, for about 15-17 minutes, or until starting to turn golden and caramelize; add chopped mushrooms and sauté, stirring occasionally, for 8-10 minutes, or until any liquid released by the mushrooms has evaporated; add garlic and bacon and continue sautéing for another 5 minutes; remove from heat and add to chicken/spinach mixture in bowl; allow to cool for about 5 minutes, then stir in heavy cream, cheeses and eggs, stirring to distribute evenly; check seasoning and season to taste (it probably won’t need much salt because of the cheeses, but lots of freshly-ground pepper will be great!).
Preheat oven to 375°F.
Butter your one large pan (or two smaller ones – if you are making 2 pies, separate the phyllo into 2 equal parts and cover each with a dampened kitchen towel); start staggering your sheets of phyllo into the pan, and buttering each one generously; when half the sheets have been used, spoon in the chicken/vegetable filling; cover filling with remaining phyllo sheets, buttering each one generously as it is added; fold in edges, tucking them in to make an attractive finish, and butter top layer generously, using up remainder of butter.
Score top of pastry lengthwise into 4 sections with a small, sharp knife, making sure not to cut all the way through (do NOT cut through pastry on bottom layer) – these will facilitate cutting pieces when pie is baked and will also serve as steam vents; sprinkle some cold water generously over the pie (I shake the water off my fingers, about 3 runs under the tap and shaken all over the pastry) and pop the pie into the oven.
Bake for 50-60 minutes, or until the pie is a rich golden-brown – for the last 20 minutes of baking, slip the pan directly onto the floor of the oven to brown and crisp the bottom phyllo sheets (if you are baking the frozen pie, put it in the oven frozen, tack on another 10-15 minutes baking - if phyllo is getting too brown, tent with foil).