Making Cheesy Potato Casserole is simple and enjoyable. Follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Prepare the Potatoes: Wash, peel (if desired), and dice the potatoes into uniform cubes. This ensures even cooking.
3. Boil Potatoes: In a large pot, add the diced potatoes and enough water to cover them. Bring to a boil and cook for about 10-15 minutes until just tender; do not overcook.
4. Drain Potatoes: Once tender, drain the potatoes and set aside to cool slightly.
5. Mix Ingredients: In a large bowl, combine the sour cream, cream of mushroom soup, milk, chopped onion, garlic powder, salt, and pepper. Mix well until smooth.
6. Combine Potatoes and Mixture: Gently fold the boiled potatoes into the creamy mixture until evenly coated.
7. Add Cheese: Stir in 1 ½ cups of shredded cheddar cheese until well incorporated.
8. Transfer to Baking Dish: Grease a 9×13-inch baking dish with butter. Pour the potato mixture into the prepared dish, spreading it evenly.
9. Top with Cheese and Cornflakes: Sprinkle the remaining ½ cup of cheddar cheese on top. If desired, add crushed cornflakes for added crunch.
10. Bake: Place in the preheated oven and bake for 40-50 minutes or until bubbling and golden on top.
11. Cool Slightly: Remove from the oven and let it cool for about 10 minutes before serving. This will help the casserole set.

These easy steps guide you through creating a delicious and satisfying Cheesy Potato Casserole that everyone will adore!