Preheat the oven to 375°F. Lightly grease a 9-by-13-inch casserole dish.
Combine the cashews, broth, flour, and nutritional yeast in a blender. Puree until completely smooth; this can take up to 5 minutes depending on the strength of your machine. Periodically stop the machine to prevent it from overheating, and scrape down the sides of the blender jar with a rubber spatula make sure you get everything.
Meanwhile, heat a large sauté pan over medium-high heat. Heat the olive oil, then sauté the green beans and onion with a pinch of salt until the green beans are softened, about 5 minutes. Add the mushrooms and sauté 7 to 10 minutes more. The mushrooms should have released most of their moisture.
Add the blender mixture to the pan, along with the onion powder, remaining 1 teaspoon salt, and a few grinds of black pepper. Cook, stirring often, until thickened, about 10 minutes. Add half of the fried onions.
Transfer the green bean mixture to the casserole and top with the remaining fried onions. Bake until browned and bubbly, 22 to 25 minutes. Let cool for a few minutes before serving.