Line a large skillet with a thin layer of broth and saute the onion and garlic over high heat until onion is translucent, about a minute. Reduce heat to medium and add the remaining broth plus mushrooms, green beans, poultry seasoning, Dijon mustard, thyme, sage, and a dash of nutmeg, stirring to coat the mushrooms with seasonings.
Continue to cook until the mushrooms are soft and the green beans are cooked but still crisp, about 7 minutes. Meanwhile, in a measuring glass or small bowl, whisk plant-based milk with nutritional yeast and cornstarch.
Once green beans and mushrooms are cooked, pour in the plant-based milk mixture. Stir a few times and allow the mixture to thicken. Once it thickens slightly, turn off heat. Add salt and pepper to taste, then spoon the casserole mixture into bowls and top with toasted bread crumbs and fried onions if desired.