Preheat the oven to 350°F.

Lightly grease a 9x13-inch baking dish with a bit of butter.

Arrange tater tots/potato rounds in the baking dish. Set aside.

In a large bowl, combine sour cream, cream of mushroom soup and 1/2 cup of the Parmesan cheese.

Pour mixture over the tater tots.

Sprinkle the remaining 1/2 cup of Parmesan cheese and the crushed French-fried onions on top of the casserole.

Bake for 45 to 60 minutes, or until warmed through and the top is golden brown.