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Heat a large skillet over medium-high heat. Cook bacon pieces until slightly crispy and cooked through, about 5 minutes. Remove from skillet with a slotted spoon. Set bacon aside.
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Leaving 2 tbsp of bacon fat in the skillet, add leeks and cook for 3-5 minutes, stirring occasionally until starting to soften. Add pork and mix, breaking up pork with a wooden spoon. Cook until pork is almost cooked through (no visible pink left), about 5 minutes more. Add thyme, sage, and garlic and season skillet with salt and pepper. Cook for 1-2 minutes further.
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Sprinkle flour over contents in skillet. Stir until leeks and pork are fully coated with flour. Slowly (VERY slowly, starting with 2 tbsp at a time, then slowly increasing quantity to avoid lumps) add beer, stirring constantly. Add chicken broth and bring to a boil. Reduce temperature and simmer for about 10-15 minutes until liquid has thickened slightly.
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Preheat oven to 400° F.
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Add apples and return bacon to skillet. Toss to incorporate. Remove from heat and set aside.
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On a lightly floured surface, roll out the large piece of pastry into a 12" circle (1/4" thick). Lay pastry into a 10" cast iron skillet. Place pork, apple, and ale mixture on top of pastry in cast iron skillet. Spread until reasonably even.
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Rollout smaller pastry disk to 11" circle (1/4") and set on top of pie. Trim overhang to 1" if needed. Press pastry rims together and tuck into the skillet. Brush top with beaten egg mixture and cut a small hole or slits to let the air escape.
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Place skillet on a large baking sheet (to catch any drippings) and bake for 30 minutes. Reduce heat to 375° F and bake for 40-45 more minutes.
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Remove from oven and allow to cool for at least 20 minutes. Cut into wedges and serve.