In a large bowl, combine the diced potatoes, shredded cheddar cheese, sour cream, cream of chicken soup, milk, green onions, garlic powder, onion powder, salt, and black pepper. Mix well until all ingredients are evenly coated.
Transfer the potato mixture into a greased slow cooker. Spread it out evenly.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the potatoes are tender.
If using, mix the crushed cornflakes with melted butter in a small bowl. About 30 minutes before serving, sprinkle the cornflake mixture on top of the casserole. Cover and continue cooking until the topping is golden and crispy.
Once done, serve hot and enjoy your cheesy potato casserole!