Puff Pastry Chicky Casserole

Ingredients

• 1 pound chicken thighs, boneless and skinless
• 1 large leek, sliced
• 2 medium carrots, chopped
• 2 stalks celery, chopped
• 1 cup peas
• 2 tablespoons fresh parsley, chopped
• 1 tablespoon garlic, minced
• 2 teaspoons shallot, minced
• 2 teaspoons dried tarragon
• 1 1/2 teaspoons kosher salt
• 1 teaspoon dried basil
• 1 teaspoon ground black pepper
• 2 large bay leaves
• 2 1/2 cups milk
• 4 tablespoons salted butter
• 3 tablespoons all-purpose flour
• 2/3 cup heavy whipping cream
• 1 1/2 teaspoons Dijon mustard
• 1 (17.5-ounce) package frozen puff pastry, thawed
• 1 large egg, beaten