Sauce:
Pour the oil into a small saucepan.
Add the flour and cook for 1 minute over medium heat, stirring with a whisk.
Gradually whisk in the vegetable broth until the mixture thickens.
Add the nutritional yeast, then season with salt and pepper.

Filling:
Place the diced carrots and potatoes in a saucepan.
Add enough water to cover the vegetables.
Bring to a boil. Reduce the heat and simmer for about 20 minutes.
Drain and set aside.

In a saucepan, sauté the onions and mushrooms in the oil over medium-high heat for 5 minutes.

Add the white wine and cook until it has evaporated.
Add the cooked vegetables, seitan, and sauce, then season to taste.

Reheat the vol-au-vent pastry shells in the oven at 350°F (180°C) for about 10 minutes.