Allow potatoes to thaw in the fridge overnight or spread them on a baking sheet and warm them at 200° for 20 minutes or until thawed.
If making the cream of chicken soup from scratch, make and set aside to cool. (Here's a homemade recipe.) If baking immediately, preheat the oven to 350°. Grease a 9x13 baking dish and set aside.
In a large bowl, combine the sour cream, cream of chicken soup, 6 Tbsp of melted butter, salt, pepper, and dried onion.
Add the potatoes and shredded cheese and stir to combine.
Put the cornflakes (or crackers) in a large Ziploc bag and cruch gently with your hands or a rolling pin.
Add the remaining 4 Tbsp of melted butter to the crushed cornflakes and combine well.
Spread the potato mixture in the prepared pan and sprinkle the cornflakes over the top.
If baking immediately, bake uncovered for 40-50 minutes. Otherwise, cover tightly and refrigerate or freeze.