Prepare the Vegetables
Boil the potatoes in a large pot of salted water for about 10-12 minutes, until fork-tender. Drain and set aside.
Steam the green beans for 4-5 minutes, or until slightly tender.
Make the Sauce
In a mixing bowl, combine the cream of mushroom soup, milk, melted butter, garlic powder, onion powder, salt, and pepper. Whisk until smooth.
Assemble the Casserole
Preheat your oven to 350°F (175°C).
In a greased baking dish, layer the potatoes and green beans evenly. Pour the sauce over the top, spreading it to cover the vegetables.
Sprinkle the shredded cheese over the mixture.
Bake
Cover the casserole with aluminum foil and bake for 25 minutes.
Remove the foil, top with crispy onions, and bake for an additional 10 minutes, until the onions are golden and the cheese is bubbly.
Serve
Let the casserole rest for 5 minutes before serving warm.