How to Make It
On a lightly floured work surface, roll out each puff pastry sheet to a 10-by-14-inch rectangle. Stack the pastry sheets on a cookie sheet with a piece of parchment paper between them. Refrigerate until chilled.
In a large saucepan, cover the potatoes with 1 inch of water. Add salt, bring to a boil and cook over moderately high heat until the potatoes are just tender, about 15 minutes. Drain and cool under running water. Pat the potatoes dry, then peel and quarter. Gently crush with your hand or a wooden spoon.
In a medium saucepan, boil the ale over high heat until reduced to 1/3 cup, about 5 minutes. Add the cream and cook, whisking, until reduced to 3/4 cup, about 5 minutes. Add the cheese and cook over moderate heat, stirring constantly, until the sauce is smooth, about 3 minutes. Remove from the heat and whisk in the egg yolks, Worcestershire and mustard. Season the sauce with salt and pepper and let cool. Carefully stir in the crushed potatoes and season again with salt and pepper.
Arrange 1 chilled puff pastry sheet on a rimmed baking sheet lined with parchment paper and brush with some of the beaten egg. Spread the potato filling on top, leaving a 1/2-inch border all around. Top with the second sheet of puff pastry and press the edges together to seal; crimp decoratively. Brush the top of the pie with more of the beaten egg and cut a few slits for venting. Refrigerate until firm, at least 30 minutes.
Preheat the oven to 400°. Bake the pie for 25 minutes. Reduce the oven temperature to 350°, rotate the baking sheet and bake until the pastry is browned and the filling is bubbling, 25 to 30 minutes longer. Let cool slightly before serving.