Melt butter in a skillet over medium heat.
Add onions and garlic and saute until tender, about 3-5 minutes. Set aside to cool slightly.
In the slow cooker, add sour cream, 3 cups cheese, salt, and pepper, and stir to combine.
Stir in cooled onion mixture.
Fold in potatoes.
Cover and cook on HIGH for about 3-3.5 hours.
Remove lid and top with remaining cheese, and garnish with bacon and chives if desired. Return lid until cheese has melted.
