Place onion in a microwave-safe bowl and cover with water. Microwave on high until soft, 7 to 8 minutes. Drain.
Preheat the oven to 350 degrees F (175 degrees C). Spray a 9x13-inch casserole dish with cooking spray.
Cook and stir ground beef in a large nonstick skillet over medium-high heat until browned and crumbly, 5 to 7 minutes. Drain and discard grease.
Transfer beef to the prepared casserole dish and spread onion pieces evenly over top. Spread soup over onions and sprinkle Cheddar cheese evenly over soup. Layer frozen potato nuggets in rows on cheese.
Bake in the preheated oven for 20 minutes. Lightly spray potato nuggets with cooking spray, and bake until cooked through and crunchy, about 10 minutes more.