Preheat the oven to 350 degrees F.
Add the oil to a skillet over medium heat. Add the onions and cook until soft and beginning to brown, about 15 minutes. Add the beef to the onions and cook until the meat is brown, stirring and scraping the bottom of the skillet to release the yummy brown bits, about 10 minutes. Season with salt and pepper.
Transfer the beef and onion mixture from the skillet into a 9-by-13-inch baking dish. Scatter the green beans and corn evenly on top. Spread the cream of mushroom soup over top and top with frozen potato nuggets. Bake uncovered for 1 hour; the potato nuggets should be golden brown. Serve.