Saute the green onions in 1/4 cup butter for about 3 minutes, until soft. Add the sour cream and soup and stir till combined and bubbly. Stir in the cheese until melted.
Remove from heat and add the shredded potatoes, stir to coat completely.
Pour into a lightly greased 9x13in casserole dish, set aside.
Add the crushed corn flakes with the 1/2 cup melted butter and stir to coat. Spread on top of the potatoes in an even layer.
Bake for 40-45 minutes or until center is hot and bubbly.
You can use freshly shredded potatoes in place of the frozen variety and reduce cooking time to 30 minutes.If you want to freeze the casserole, do so before adding the corn flakes. Before baking add the topping and increase baking time till the center is hot and bubbly.