Preheat oven to 350°F and spray a 9x13-inch baking dish with cooking spray.
Cook the ground beef and diced onion in a large skillet over medium-high heat, breaking up the meat, until the beef reaches 160°F (71°C), about 8 to 10 minutes. Drain excess fat.
In the prepared baking dish, combine the cooked beef and onion, frozen hash browns, both soups, sour cream, 2 cups cheddar, frozen vegetables, garlic powder, onion powder, salt, and pepper; spread evenly.
Cover with aluminum foil and bake for 35 minutes.
Remove the foil and sprinkle the remaining 1 cup of cheese on top, followed by the crushed cornflakes or French fried onions.
Return to the oven uncovered and bake for an additional 10-15 minutes, until the topping is golden and crispy.
Let the casserole rest for 5 minutes before serving to allow it to set up properly.