Use non-stick cooking spray to lightly grease your slow cooker insert. Dump all of the ingredients, except for the toppings, into the greased slow cooker.
Stir until combined and no sour cream streaks remain. Cover with the lid and cook on LOW for 5 hours or HIGH for 3 hours.
15 minutes before serving, top with shredded cheddar and green onions (if using). Serve once the cheese topping is good and melty.