- In a skillet over medium-high heat, brown the onion and mushrooms in the oil. Season with salt and pepper. Set aside.
- In a saucepan over medium heat, melt the butter. Add the flour and cook for 2 minutes, whisking constantly. Whisk in the broth and bring to a boil. Add the sweet potatoes, celery, beans and mushroom mixture. Bring to a boil and simmer gently for 10 to 15 minutes, stirring frequently, until the vegetables are tender. Add the chicken and adjust the seasoning. Transfer to a bowl and let cool. Cover and refrigerate for 1 hour.
- Meanwhile, choose an 8-cup (2 litres) baking dish (a soufflé dish, for example). Roll out the dough so that the diameter is about 1 1/2inches (4 cm) larger than the dish. Place the dough on a baking sheet and refrigerate for 30 minutes.
- With the rack in the middle position, preheat the oven to 400°F (200°C).
- Spoon the chicken mixture into the dish. Cover with the dough and press on the rim of the dish. Using a pastry brush, cover the dough with beaten egg.
- Bake for 40 to 45 minutes or until the pastry is golden brown. Let rest for 5 minutes before serving.
You can also make individual pot pies using 6 heatproof bowls with a capacity of about 10 oz (300 ml) (ramekins, onion soup bowls, etc.). In this case, you will need an extra 7 oz (200 g) of puff pastry, and the cooking time will be approximately 30 minutes.