Align the green beans with the stem facing in one direction. Cut off the stems. (Green beans may be cut in half if desired.)
Add green beans to a large pot. Cover completely with water. Bring to a boil and cook until tender, about 15 minutes. Drain beans and transfer to a casserole dish.
Clean and slice mushrooms. Finely chop 1/2 the large onion. (Use the other half of onion to prepare homemade onion strings.)
In a skillet, heat 2 Tbsp oil. Saute mushrooms with onions until golden.
Slice the other 1/2 of onion, vertically and prepare fried onion strings. optional.
Add the mushrooms/onions to the green beans and mix well. Lightly season with salt and pepper.
In a small bowl combine all the ingredients for the sauce. Add to the green beans. Mix.
Sprinkle the cheese on over everything. Add fried onions to the edges of the dish.
Cover the baking dish with foil.
Bake covered for 25 minutes in a preheated oven to 375° F. Remove foil, bake uncovered an additional 10 minutes until cream bubbles and cheese completely melts.