Place the cubed potatoes, sliced sausage, and diced onion in the slow cooker. Add the salt and black pepper, then stir to combine.
Sprinkle 1 1/2 cups of the shredded cheddar cheese over the potato mixture and stir again so the cheese is distributed.
Cover and cook on low for 6 hours or on high for 3 hours, until the potatoes are tender when pierced with a fork.
Stir in the sour cream until it is fully mixed and the casserole looks creamy.
Sprinkle the remaining 1/2 cup of cheddar cheese on top, then cover and cook for 10 more minutes until the cheese melts.
Turn off the slow cooker. Top with the crumbled bacon and sliced green onions before serving.