If your favorite part of pot pie is the crust and you’re vegetarian, this is the dish for you. A creamy sauce is poured over hearty root vegetables like carrots, parsnips and sweet potatoes and topped with layers of puff pastry. It takes less than 30 minutes to whip up, making it perfect for a weeknight dinner with just a few dishes to clean!
Preheat the oven to 400 degrees F. Spray a half sheet pan (18-by-13-inches) with cooking spray.
Put the butter, thyme, cinnamon and allspice in a small saucepan over medium heat and cook until the butter melts, about 1 minute. Whisk in the flour and cook, stirring constantly, until the mixture is a sandy consistency, about 1 minute. Pour in the vegetable broth, whisking constantly, until smooth. Stir in 2 teaspoons salt and a few grinds of pepper and bring to a boil, about 2 minutes. Reduce the heat to medium-low and simmer gently until thickened and the flour flavor is gone, about 10 minutes.
Meanwhile, put the sweet potatoes, carrots, parsnips, celery, pearl onions, 1 teaspoon salt and a few grinds of pepper on the prepared half sheet pan and toss to combine. Set aside.
Stack the 4 puff pastry sheets on top of each other on a lightly floured surface. Roll out the puff pastry to a 20-by-15-inch rectangle. Pour the gravy on top of the vegetables on the sheet pan and toss together to coat. Place the puff pastry evenly over the filling so there is about a 1-inch overhang on the edges. Trim the overhang with kitchen shears, making sure that the filling stays completely covered. Using the shears, make 12 small X marks all over the pastry; these slits will help steam escape. Beat the egg with a splash of water in a small bowl. Brush the pastry with the egg wash and bake until deep golden brown, 35 to 40 minutes. Let the pot pie cool for 10 minutes, then slice and serve.