Preheat oven to 350°F (175°C)
In a large bowl, combine:
2 cans (10.5 oz each) condensed cream of mushroom soup
1 can (4 oz) sliced mushrooms, drained (optional)
1/2 to 3/4 cup milk (use 1/2 cup for thicker, up to 3/4 cup for looser)
1/2 teaspoon salt (optional, to taste)
1/4 teaspoon black pepper
Add 4 to 5 cups cooked green beans (fresh steamed or thawed frozen), and stir to coat
Transfer mixture to a greased 2-quart baking dish
Bake uncovered for 20 minutes
Sprinkle 2 cups French-fried onions over the top
Bake 5 to 10 minutes more until hot and lightly browned
Rest 5 minutes before serving