Cut the bacon in small sticks thick as a matchstick and as wide as a cigarette is thick Fry the bacon with no added fat over medium-high heat until it becomes soft. Reserve.
Wash the leek and finely chop it. Peel the onions and slice them thinly. Fry onions and leek in the bacon's remaining fat until they are soft and start turning brown.The author at FX says I can't emphasize enough how important is this browning process. If you add every ingredient raw to the pie it will just boil and you'll miss half the flavor. .
Start by peeling the cooked potatoes and grate them coarsely. Salt.
Peel, core and dice pears and apples. Save this for last lest they turn brown.
Dice the cheese, heat the oven to 375 and prepare your ingredients around a large baking tray with parchment paper on it.
Spread the puff pastry onto a baking sheet. Spread the potatoes evenly on the pastry, leaving a good 2.5/1'' space all around.
On this bed of potatoes, lay the pear and apple cubes and cover them with the onion and leeks and then the bacon.
Finish with the diced cheese and sprinkle with crushed black pepper.
Cover with the second puff pastry sheet and carefully seal around the edges. Make a few holes on top with a fork to let the steam escape.
Beat the egg yolk or whole egg and paint the pie with a kitchen brush. This will turn it golden when it bakes.
Bake for 30 minutes in a hot oven (190°C) until golden.
Let it rest for 5 minutes before slicing with a long and very sharp serrated blade.