Spray the inside of a 6 quart slow cooker liberally with cooking spray.
In a large mixing bowl, whisk together the butter, cream of potato soup, cream of celery soup, sour cream, chicken broth, ranch dressing mix, garlic salt or seasoned salt and black pepper.
Add the thawed hash brown potatoes, diced onion and 2 ½ cup shredded Colby-Jack cheese. Mix until the potatoes are evenly coated. Pour Into the slow cooker.
Cover the opening of the slow cooker with paper towels to absorb the condensation, then fit the lid tightly on top. The paper towels will prevent the condensation from dripping back into the potatoes making the sauce too thin.
Cook on low for 5-6 hours or until the potatoes are cooked through and fork tender.
Uncover and sprinkle with the remaining shredded cheese. Once done, leave uncovered and let stand for 20 minutes prior to serving. Garnish with thinly sliced green onion.