To make your Ham and Potato Casserole, follow these simple steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C).
2. Prepare Baking Dish: Grease a 9×13-inch baking dish with cooking spray or butter.
3. Boil Potatoes: In a pot, bring salted water to a boil. Add diced potatoes and cook until tender, about 10 minutes. Drain and set aside.
4. Mix Ingredients: In a large bowl, combine cooked ham, diced onions, cream of mushroom soup, sour cream, shredded cheese, garlic powder, salt, pepper, and milk. Stir until well mixed.
5. Add Potatoes: Gently fold in the cooked potatoes (and peas, if using) until evenly coated with the mixture.
6. Transfer to Baking Dish: Pour the mixture into the prepared baking dish, spreading it evenly.
7. Bake: Cover with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and bubbly.
8. Cool and Serve: Remove from the oven and let it cool for about 5 minutes before serving.

These clear steps will help ensure you create a mouthwatering Ham and Potato Casserole, perfect for any occasion!