Meatless Mushroom Tart

Step 1
Preheat oven to 400 degrees F (200 degrees C). Lightly grease a baking sheet.

Step 2
Line a 10-inch pie dish with 1 sheet of puff pastry. Trim the edge so 1 inch hangs over the rim of the dish.

Step 3
Bake in preheated oven just long enough to set the pastry, 5 to 10 minutes.

Step 4
Cut the second sheet of puff pastry into 1 inch wide strips. Place the strips on the prepared baking sheet. Refrigerate the strips and the baked pie shell for at least 30 minutes, or up to 24 hours.

Step 5
Meanwhile, melt the butter in a skillet over medium-high heat. Stir in the onions, and cook until soft and transparent, about 6 minutes. Stir in the mushrooms and thyme, and cook until the mushrooms soften and release their juices, about 6 minutes. Mix the cream cheese into the mushroom mixture, stirring until melted. Season to taste with salt and pepper.

Step 6
Spoon the mushroom mixture into the pie shell. Arrange the pastry strips in a lattice pattern across the filling. Fold and crimp the ends of the strips together with the pie shell edge trimming any necessary pastry to make an even edge.

Step 7
Whisk the egg yolk and milk together in a small bowl, and use to brush the top of the pie.

Step 8
Bake in preheated oven until the crust is golden brown and the filling is heated through, up to 45 minutes. Check for doneness after 30 minutes. Cool another 30 minutes before serving.