Preheat your oven to 350°F (175°C).

Peel and dice the potatoes into small cubes.

In a large mixing bowl, combine the diced potatoes, cheddar cheese, sour cream, cream of mushroom soup, milk, green onions, garlic powder, salt, and pepper. Stir until everything is evenly mixed.

Pour the potato mixture into a greased 9x13 inch baking dish.

If desired, sprinkle the crushed cornflakes over the top of the casserole.

Place the baking dish in the preheated oven and bake for 45 minutes.

After 45 minutes, check if the potatoes are tender by inserting a fork. If they are still firm, continue baking and check every 5 minutes.

Once done, remove the casserole from the oven and let it sit for a few minutes before serving.