When the fries are nearly done, make the mushroom gravy: Combine 1 ¼ cups of water, bouillon cubes, mushrooms, and soy sauce in a small saucepan and bring to a slow boil. Turn down the heat and simmer until the mushrooms are wilted, about 5 minutes.
Meanwhile, combine the cornstarch with just enough water to make it smooth and pourable.
Slowly whisk the dissolved cornstarch into the simmering broth, stirring constantly with a whisk, until the mixture is thickened.
Remove from the heat and whisk in the Italian seasoning and the nutritional yeast, if using. Transfer about half of the gravy to a spouted container and reserve the rest in the pan.