Preheat oven to 350°F; spray a 9x13-inch dish.
Cook beef and onion in large skillet over medium-high, breaking up meat, about 8 to 10 minutes until beef reaches 160°F (71°C). Drain fat.
Combine cooked beef and onion, hash browns, soups, sour cream, 2 cups cheddar, vegetables, garlic powder, onion powder, salt, and pepper in dish; spread evenly.
Cover with foil; bake 35 minutes.
Uncover; top with remaining 1 cup cheese, then cornflakes or fried onions.
Bake uncovered 10-15 minutes until topping is golden and crisp.
Rest 5 minutes to set before serving.