Preheat the oven to 350F

Coarsely chop the onion

Melt 1/4 cup of the butter in a large skillet over medium heat

Add the onions to the pre-heated skillet

Cook, stirring frequently, for 3 minutes

In a large mixing bowl combine the onions, soup, sour cream, and cheese

Mix well

Add the potatoes to the mixture

Stir until the potatoes are coated in the other ingredients

Spray the 9x13 baking dish with cooking spray

Pour the mixture into the baking dish

Add the remaining butter to the large skillet

Melt the butter

Add the Frosted Flakes

Mix until the Frosted Flakes are coated in butter

Spread the Frosted Flakes over the mixture in the baking dish

Cover with foil

Bake for 45 minutes in the preheated oven

Remove the foil

Bake an additional 15 minutes

Serve