Saute the onions with butter in a large cast iron frying pan or any large pan over medium heat until the onions soften and become transparent.
Add sour cream and can of soup (10-11 fluid oz.) to your pan. Whisk to combine and bring to a simmer. This should take about 5 minutes.
Whisk in the shredded cheddar cheese until cheese is nicely combined into the sauce.
Whisk in the seasoned salt, pepper, and garlic powder.
Give it a taste. Adjust the salt, cheese, pepper, and garlic level to your taste. If you want more tang, add more sour cream.
Check the consistency. If the sauce is too thick, it will be hard to combine with the potatoes. It should be like a thicker gravy. If you think it's too thick, add 3 tablespoons of milk at a time until it's the right consistency.
Dump hash brown potatoes into a large bowl. Pour the cheese sauce over the hash browns. Mix them gently until all the potatoes are coated evenly with the sauce.
Coat a 9×13 cake pan with cooking spray or your favorite oil on a paper towel.
Spread the potatoes evenly in the cake pan.
Cover the potatoes with foil and bake in a 325-350° oven for 1 hour or until the temperature reaches 160°. If you'd like them more brown on top, remove the foil about 15 minutes before they are done baking.
Optional: add fresh or dried parsley to the top for color