Wash hands with soap and water. Preheat the oven to 400 degrees. Wash and dry the produce. Peel and chop the onion. Chop the celery. Grease a large pie plate or casserole dish.

In a large pot, heat 2 tbs butter over medium-high heat. Add the onion and celery. Cook until tender, 7 to 9 minutes. Stir in the mushrooms and garlic, and continue to cook until all the vegetables are tender and beginning to turn golden, 5 to 6 minutes more.

Add the flour and spice blend to the pot, and stir to combine. Cook for 1 to 2 minutes, or until the flour emits a toasty aroma. Add the broth gradually, mixing constantly to make sure there are no lumps.

Bring the mixture to a simmer and cook for 10-15 minutes or until thickened. Stir in the chicken and frozen vegetables, and bring to a simmer. Stir in the cream and bring to a simmer. Simmer 3 to 4 minutes, or until the stew has developed good flavor. Ladle the chicken mixture into the prepared pie plate.

Press or roll the two sheets of puff pastry together to make one big sheet. Transfer to the top of the skillet and trim away the excess with scissors (the heat of the filling will make the puff pastry soften, so be sure to press the pastry around the edge of the pan to seal it).

In a small bowl, whisk the egg with 1 tbs water. Brush the egg wash over the puff pastry. Use a paring knife to cut a ½-inch-long vent on the top of the potpie.

Bake until the puff pastry is golden brown and the filling is bubbling, 15 to 20 minutes. Let cool for 5 to 10 minutes before serving. This recipe makes 4 servings. Refrigerate leftovers within 2 hours.