Blanch the green beans in boiling salted water for 5 minutes. Drain well.
In a large bowl, combine the drained green beans, condensed cream of mushroom soup, milk, and 0.5 cup of fried onions. Season with black pepper.
Pour the mixture into a 9x13 inch baking dish.
Bake for 20 minutes.
Remove from oven and top with the remaining 0.5 cup of fried onions.
Return to oven and bake for an additional 10 minutes, or until the onions are golden brown and the casserole is bubbly.
Notes
For an extra creamy casserole, you can add a splash of heavy cream to the soup mixture. You can also add a pinch of nutmeg for a subtle flavor enhancement.