Let the potatoes completely thaw, and then put them on some paper towel to remove the moisture.
Put the potatoes in a casserole pan. Add the chopped onion.
Mix the sour cream, cheese, and soup together in a large bowl.
Put the mixture on top of the potato mixture, and with your hands, mix the whole thing up. Flatten it without crushing it.
Put the asparagus, mushrooms, and broccoli florets on top of that mixture, nicely.
Carefully mix the corn flakes and Parmesan together, then carefully spread them on the casserole.
Pour the melted butter on the bread crumb mixture.
Bake for about 40 minutes or until the cheese has slightly browned and the casserole is bubbling. Let the casserole cool for about 10 minutes before serving.
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