Chicken Pot Pie with Mushrooms and Brie

Ingredient:

• 1 (8 1/2-ounce) double crème Brie cheese round
• 1 refrigerated pie crust (from 1 [14-ounce] package, such as Pillsbury)
• 1 tablespoon grapeseed oil
• 12 ounces mixed sliced wild mushrooms (about 6 cups, such as cremini, oyster, and shiitake)
• 2 tablespoons unsalted butter
• 1 yellow onion, chopped (about 1 1/2 cups)
• 2 carrots, peeled and chopped (about 1 cup)
• 1 celery stalk, chopped (about 1/3 cup)
• 2 garlic cloves, minced
• 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried
• 2 tablespoons dry sherry
• 1/4 cup all-purpose flour
• 3 cups chicken broth
• 1 tablespoon Dijon mustard
• 1/2 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon black pepper
• 2 cups packed shredded cooked chicken (about 14 ounces)
• 1 large egg, beaten