Arrange the green beans so that the stems are all facing the same direction, then trim off the stems. (If preferred, you can cut the green beans in half.) Place the green beans in a large pot and cover them completely with water. Bring to a boil and cook until tender, about 15 minutes. Drain the beans and transfer them to a casserole dish.
Prepare a batch of crispy onion rings.
Clean and slice the mushrooms, and finely chop the onion. Heat 2 tablespoons of oil in a skillet, and sauté the mushrooms and onions until they turn golden.
Add the cooked mushrooms and onions to the green beans and mix well. Season lightly with salt and pepper. In a small bowl, combine all the ingredients for the sauce, then stir it into the casserole dish with the green beans.
Sprinkle cheese over the top and arrange the fried onions around the edges of the dish. Cover the casserole with foil.
Bake in a preheated 375°F oven for 25 minutes, then remove the foil and bake uncovered for an additional 10 minutes, or until the cream is bubbling and the cheese has melted completely. Enjoy!