Preheat your oven to 350°F (175°C).
In a large pot, bring salted water to a boil. Add the diced potatoes and cook for about 10 minutes until just tender.
Add the green beans to the pot and cook for another 5 minutes. Drain the vegetables and set them aside.
In a mixing bowl, combine the cream of mushroom soup, chopped onion, garlic powder, salt, and black pepper. Mix well.
In the baking dish, combine the cooked potatoes and green beans. Pour the soup mixture over the vegetables and stir until well coated.
Sprinkle shredded cheddar cheese evenly over the top.
In another bowl, mix the breadcrumbs with olive oil until the breadcrumbs are coated. Sprinkle the breadcrumb mixture over the cheese layer.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is bubbling around the edges.
Remove from the oven and let it cool for a few minutes before serving.