Preheat the oven to 375 degrees F (190 degrees C).
Heat a large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain grease. Add onion and cook until soft and translucent, about 5 minutes.
Meanwhile, combine condensed soup, sour cream, and green bean liquid in a small bowl. Set sauce aside.
Add green beans to the cooked beef and onions; mix to combine and transfer to a casserole dish. Top with sliced cheese and cover with frozen potato nuggets. Pour sauce over top.
Bake in the preheated oven potatoes are browned and sauce is bubbly, about 1 hour.
