Soak dried porcini in 1 cup boiling water for 20 minutes. Strain and chop; reserve the soaking liquid (avoid gritty sediment).
Heat 1 tablespoon oil in a Dutch oven over medium-high heat. Working in batches, sear the beef until deeply browned. Transfer to a bowl and set aside.
In the same pot, cook diced bacon until crisp. Add mushrooms and cook for 4–5 minutes, until browned. Transfer both to a plate. Reserve 2 tablespoons bacon fat in the pot.
Lower the heat to medium. In the reserved bacon fat (add extra oil if needed), sauté garlic for 1 minute. Add onions and carrots; cook 5–6 minutes until starting to soften and lightly color.
Add chopped porcini (if using), brown sugar, and flour. Cook for 1–2 minutes, stirring to coat the veggies and cook off the flour taste.
Return the beef and its juices to the pot. Stir in the Guinness, beef stock, and reserved mushroom soaking liquid. Season with salt and pepper. Add thyme, bay leaves, and chopped parsley.
Cover and bake at 325°F for about 2 hours, or until the beef is fork-tender.
Remove the pot from the oven. Stir in the cooked bacon and mushrooms. Let the filling cool completely before assembling the pie (this keeps the pastry from getting soggy).
Preheat Oven
Preheat to 425°F. Place a baking sheet on the middle rack to catch any drips.
Grease a 10-inch pie dish or 9x9-inch baking dish.
Spoon the cooled beef mixture into the dish, leaving behind excess liquid if it's too soupy. The filling should mound slightly above the rim.
Lay the puff pastry sheet over the filling. Trim any overhang, crimp the edges, and brush with egg yolk.
Cut a few slits in the top for steam. Place the pie on the hot baking sheet and bake for 35–40 minutes, or until the pastry is deep golden brown and crisp.
Notes
Erren’s Top Tips
Brown the beef in batches so it sears instead of steams.
If the gravy tastes slightly bitter from the stout, add 1 teaspoon red wine vinegar to brighten the flavor.
Let the filling cool completely before adding the puff pastry. This keeps the pastry from getting soggy and allows it to puff and crisp properly in the oven.
Bake the pie on a preheated baking sheet to help the bottom of the pastry cook evenly.
If the filling seems too loose after braising, simmer it uncovered on the stovetop until thick enough to coat the back of a spoon.
Storage & Freezing
Store leftover pie covered in the refrigerator for up to 4 days.To safely chill the filling before assembling or storing, allow the cooked beef mixture to cool at room temperature for about 30 minutes, then transfer it to shallow containers so it cools quickly in the refrigerator. Avoid leaving cooked beef out longer than 2 hours.Reheat leftover pie in a 350°F oven until heated through so the pastry stays crisp.The filling can also be frozen without the pastry for up to 3 months. Thaw overnight in the refrigerator before topping with fresh puff pastry and baking.
Variations & Add-Ins
Stir in 1 cup frozen pearl onions during the last 30 minutes of braising for extra sweetness and texture.Use mashed potatoes instead of puff pastry for a cozy cottage pie-style finish.Add parsnips or turnips for extra depth and a rustic feel.Divide the filling into ramekins and top with pastry rounds to make individual pies.