Preheat oven to 175 °C (≈350 °F).
If using fresh green beans: blanch them in boiling water for about 5 minutes until tender-crisp; then drain. (If using canned/frozen beans, just make sure they are drained/thawed.)
In a mixing bowl, stir together the condensed cream-of-mushroom soup, milk, salt and pepper until smooth. Then add the green beans and mix until beans are evenly coated.
Transfer the mixture into a greased baking dish and spread evenly.
Sprinkle a generous layer of crispy fried onions on top (optionally reserving some to add mid-bake) and bake for about 20-25 minutes, or until the casserole is hot and bubbly and onions are golden brown.
Remove from oven and let sit a few minutes before serving. Serve warm as a side dish.