Preheat the oven to 350°F.
In a large mixing bowl, combine the cream of mushroom soup, milk, garlic powder, salt, and black pepper. Stir until well mixed.
In a greased 9x13-inch baking dish, layer half of the sliced potatoes on the bottom.
Add half of the chopped onion and half of the green beans.
Pour half of the soup mixture over the vegetables, then sprinkle with half of the cheddar cheese.
Repeat the layers with the remaining potatoes, onions, green beans, soup mixture, and cheese.
Cover the baking dish with aluminum foil and bake for 45 minutes.
Remove the foil and sprinkle the French fried onions on top.
Bake for an additional 15 minutes, or until the potatoes are tender and the casserole is bubbly and golden brown.
Let the casserole cool for about 10 minutes before serving.